diff --git a/db/db_bench.cc b/db/db_bench.cc
index 623c72ecb..3f38b3716 100644
--- a/db/db_bench.cc
+++ b/db/db_bench.cc
@@ -87,6 +87,16 @@ static bool FLAGS_histogram = false;
 // (initialized to default value by "main")
 static int FLAGS_write_buffer_size = 0;
 
+// The number of in-memory memtables. 
+// Each memtable is of size FLAGS_write_buffer_size.
+// This is initialized to default value of 2 in "main" function.
+static int FLAGS_max_write_buffer_number = 0;
+
+// The maximum number of concurrent background compactions
+// that can occur in parallel.
+// This is initialized to default value of 1 in "main" function.
+static int FLAGS_max_background_compactions = 0;
+
 // Number of bytes to use as a cache of uncompressed data.
 // Negative means use default settings.
 static long FLAGS_cache_size = -1;
@@ -159,6 +169,9 @@ static int FLAGS_level0_stop_writes_trigger = 12;
 // Number of files in level-0 that will slow down writes.
 static int FLAGS_level0_slowdown_writes_trigger = 8;
 
+// Number of files in level-0 when compactions start
+static int FLAGS_level0_file_num_compaction_trigger = 4;
+
 // Ratio of reads to writes (expressed as a percentage)
 // for the ReadRandomWriteRandom workload. The default
 // setting is 9 gets for every 1 put.
@@ -326,7 +339,8 @@ class Stats {
       } else {
         double now = FLAGS_env->NowMicros();
         fprintf(stderr,
-                "... thread %d: %ld ops in %.6f seconds and %.2f ops/sec\n",
+                "%s thread %d: %ld ops in %.6f seconds and %.2f ops/sec\n",
+                FLAGS_env->TimeToString((uint64_t) now/1000000).c_str(),
                 id_,
                 done_ - last_report_done_,
                 (now - last_report_finish_) / 1000000.0,
@@ -873,6 +887,8 @@ class Benchmark {
     options.create_if_missing = !FLAGS_use_existing_db;
     options.block_cache = cache_;
     options.write_buffer_size = FLAGS_write_buffer_size;
+    options.max_write_buffer_number = FLAGS_max_write_buffer_number;
+    options.max_background_compactions = FLAGS_max_background_compactions;
     options.block_size = FLAGS_block_size;
     options.filter_policy = filter_policy_;
     options.max_open_files = FLAGS_open_files;
@@ -887,6 +903,8 @@ class Benchmark {
     options.max_bytes_for_level_multiplier =
         FLAGS_max_bytes_for_level_multiplier;
     options.level0_stop_writes_trigger = FLAGS_level0_stop_writes_trigger;
+    options.level0_file_num_compaction_trigger = 
+        FLAGS_level0_file_num_compaction_trigger;
     options.level0_slowdown_writes_trigger =
       FLAGS_level0_slowdown_writes_trigger;
     options.compression = FLAGS_compression_type;
@@ -1172,7 +1190,10 @@ class Benchmark {
 
 int main(int argc, char** argv) {
   FLAGS_write_buffer_size = leveldb::Options().write_buffer_size;
+  FLAGS_max_write_buffer_number = leveldb::Options().max_write_buffer_number;
   FLAGS_open_files = leveldb::Options().max_open_files;
+  FLAGS_max_background_compactions = 
+    leveldb::Options().max_background_compactions;
   // Compression test code above refers to FLAGS_block_size
   FLAGS_block_size = leveldb::Options().block_size;
   std::string default_db_path;
@@ -1203,6 +1224,10 @@ int main(int argc, char** argv) {
       FLAGS_value_size = n;
     } else if (sscanf(argv[i], "--write_buffer_size=%d%c", &n, &junk) == 1) {
       FLAGS_write_buffer_size = n;
+    } else if (sscanf(argv[i], "--max_write_buffer_number=%d%c", &n, &junk) == 1) {
+      FLAGS_max_write_buffer_number = n;
+    } else if (sscanf(argv[i], "--max_background_compactions=%d%c", &n, &junk) == 1) {
+      FLAGS_max_background_compactions = n;
     } else if (sscanf(argv[i], "--cache_size=%ld%c", &l, &junk) == 1) {
       FLAGS_cache_size = l;
     } else if (sscanf(argv[i], "--block_size=%d%c", &n, &junk) == 1) {
@@ -1281,6 +1306,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i],"--level0_slowdown_writes_trigger=%d%c",
         &n, &junk) == 1) {
       FLAGS_level0_slowdown_writes_trigger = n;
+    } else if (sscanf(argv[i],"--level0_file_num_compaction_trigger=%d%c",
+        &n, &junk) == 1) {
+      FLAGS_level0_file_num_compaction_trigger = n;
     } else if (strncmp(argv[i], "--compression_type=", 19) == 0) {
       const char* ctype = argv[i] + 19;
       if (!strcasecmp(ctype, "none"))
@@ -1309,6 +1337,10 @@ int main(int argc, char** argv) {
     }
   }
 
+  // The number of background threads should be at least as much the
+  // max number of concurrent compactions.
+  FLAGS_env->SetBackgroundThreads(FLAGS_max_background_compactions);
+
   // Choose a location for the test database if none given with --db=<path>
   if (FLAGS_db == NULL) {
       leveldb::Env::Default()->GetTestDirectory(&default_db_path);
diff --git a/db/db_impl.cc b/db/db_impl.cc
index 07ff09688..530d25a7a 100644
--- a/db/db_impl.cc
+++ b/db/db_impl.cc
@@ -18,6 +18,7 @@
 #include "db/log_reader.h"
 #include "db/log_writer.h"
 #include "db/memtable.h"
+#include "db/memtablelist.h"
 #include "db/table_cache.h"
 #include "db/version_set.h"
 #include "db/write_batch_internal.h"
@@ -67,6 +68,7 @@ struct DBImpl::CompactionState {
     InternalKey smallest, largest;
   };
   std::vector<Output> outputs;
+  std::list<uint64_t> allocated_file_numbers;
 
   // State kept for output being generated
   WritableFile* outfile;
@@ -133,20 +135,19 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
       db_lock_(NULL),
       shutting_down_(NULL),
       bg_cv_(&mutex_),
-      mem_(new MemTable(internal_comparator_)),
-      imm_(NULL),
+      mem_(new MemTable(internal_comparator_, NumberLevels())),
       logfile_(NULL),
       logfile_number_(0),
       log_(NULL),
       tmp_batch_(new WriteBatch),
-      bg_compaction_scheduled_(false),
+      bg_compaction_scheduled_(0),
       bg_logstats_scheduled_(false),
       manual_compaction_(NULL),
       logger_(NULL),
       disable_delete_obsolete_files_(false),
-      delete_obsolete_files_last_run_(0) {
+      delete_obsolete_files_last_run_(0),
+      delayed_writes_(0) {
   mem_->Ref();
-  has_imm_.Release_Store(NULL);
 
   env_->GetAbsolutePath(dbname, &db_absolute_path_);
   stats_ = new CompactionStats[options.num_levels];
@@ -190,7 +191,7 @@ DBImpl::~DBImpl() {
 
   delete versions_;
   if (mem_ != NULL) mem_->Unref();
-  if (imm_ != NULL) imm_->Unref();
+  imm_.UnrefAll();
   delete tmp_batch_;
   delete log_;
   delete logfile_;
@@ -476,7 +477,7 @@ Status DBImpl::RecoverLogFile(uint64_t log_number,
     WriteBatchInternal::SetContents(&batch, record);
 
     if (mem == NULL) {
-      mem = new MemTable(internal_comparator_);
+      mem = new MemTable(internal_comparator_, NumberLevels());
       mem->Ref();
     }
     status = WriteBatchInternal::InsertInto(&batch, mem);
@@ -492,7 +493,7 @@ Status DBImpl::RecoverLogFile(uint64_t log_number,
     }
 
     if (mem->ApproximateMemoryUsage() > options_.write_buffer_size) {
-      status = WriteLevel0Table(mem, edit, NULL);
+      status = WriteLevel0TableForRecovery(mem, edit);
       if (!status.ok()) {
         // Reflect errors immediately so that conditions like full
         // file-systems cause the DB::Open() to fail.
@@ -504,7 +505,7 @@ Status DBImpl::RecoverLogFile(uint64_t log_number,
   }
 
   if (status.ok() && mem != NULL) {
-    status = WriteLevel0Table(mem, edit, NULL);
+    status = WriteLevel0TableForRecovery(mem, edit);
     // Reflect errors immediately so that conditions like full
     // file-systems cause the DB::Open() to fail.
   }
@@ -514,8 +515,7 @@ Status DBImpl::RecoverLogFile(uint64_t log_number,
   return status;
 }
 
-Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
-                                Version* base) {
+Status DBImpl::WriteLevel0TableForRecovery(MemTable* mem, VersionEdit* edit) {
   mutex_.AssertHeld();
   const uint64_t start_micros = env_->NowMicros();
   FileMetaData meta;
@@ -537,8 +537,8 @@ Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
       (unsigned long long) meta.file_size,
       s.ToString().c_str());
   delete iter;
-  pending_outputs_.erase(meta.number);
 
+  pending_outputs_.erase(meta.number);
 
   // Note that if file_size is zero, the file has been deleted and
   // should not be added to the manifest.
@@ -546,9 +546,6 @@ Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
   if (s.ok() && meta.file_size > 0) {
     const Slice min_user_key = meta.smallest.user_key();
     const Slice max_user_key = meta.largest.user_key();
-    if (base != NULL) {
-      level = base->PickLevelForMemTableOutput(min_user_key, max_user_key);
-    }
     edit->AddFile(level, meta.number, meta.file_size,
                   meta.smallest, meta.largest);
   }
@@ -560,37 +557,113 @@ Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
   return s;
 }
 
-Status DBImpl::CompactMemTable() {
+
+Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
+                                uint64_t* filenumber) {
   mutex_.AssertHeld();
-  assert(imm_ != NULL);
+  const uint64_t start_micros = env_->NowMicros();
+  FileMetaData meta;
+  meta.number = versions_->NewFileNumber();
+  *filenumber = meta.number;
+  pending_outputs_.insert(meta.number);
+  Iterator* iter = mem->NewIterator();
+  Log(options_.info_log, "Level-0 flush table #%llu: started",
+      (unsigned long long) meta.number);
 
-  // Save the contents of the memtable as a new Table
-  VersionEdit edit(NumberLevels());
   Version* base = versions_->current();
   base->Ref();
-  Status s = WriteLevel0Table(imm_, &edit, base);
+  Status s;
+  {
+    mutex_.Unlock();
+    s = BuildTable(dbname_, env_, options_, table_cache_, iter, &meta);
+    mutex_.Lock();
+  }
   base->Unref();
 
+  Log(options_.info_log, "Level-0 flush table #%llu: %lld bytes %s",
+      (unsigned long long) meta.number,
+      (unsigned long long) meta.file_size,
+      s.ToString().c_str());
+  delete iter;
+
+  // re-acquire the most current version
+  base = versions_->current();
+
+  // There could be multiple threads writing to its own level-0 file.
+  // The pending_outputs cannot be cleared here, otherwise this newly
+  // created file might not be considered as a live-file by another
+  // compaction thread that is concurrently deleting obselete files.
+  // The pending_outputs can be cleared only after the new version is
+  // committed so that other threads can recognize this file as a 
+  // valid one.
+  // pending_outputs_.erase(meta.number);
+
+  // Note that if file_size is zero, the file has been deleted and
+  // should not be added to the manifest.
+  int level = 0;
+  if (s.ok() && meta.file_size > 0) {
+    const Slice min_user_key = meta.smallest.user_key();
+    const Slice max_user_key = meta.largest.user_key();
+    // if we have more than 1 background thread, then we cannot
+    // insert files directly into higher levels because some other
+    // threads could be concurrently producing compacted files for
+    // that key range.
+    if (base != NULL && options_.max_background_compactions <= 1) {
+      level = base->PickLevelForMemTableOutput(min_user_key, max_user_key);
+    }
+    edit->AddFile(level, meta.number, meta.file_size,
+                  meta.smallest, meta.largest);
+  }
+
+  CompactionStats stats;
+  stats.micros = env_->NowMicros() - start_micros;
+  stats.bytes_written = meta.file_size;
+  stats_[level].Add(stats);
+  return s;
+}
+
+Status DBImpl::CompactMemTable(bool* madeProgress) {
+  mutex_.AssertHeld();
+  assert(imm_.size() != 0);
+
+  if (!imm_.IsFlushPending()) {
+    Log(options_.info_log, "Memcompaction already in progress");
+    Status s = Status::IOError("Memcompaction already in progress");
+    return s;
+  }
+
+  // Save the contents of the earliest memtable as a new Table
+  // This will release and re-acquire the mutex.
+  uint64_t file_number;
+  MemTable* m = imm_.PickMemtableToFlush();
+  if (m == NULL) {
+    Log(options_.info_log, "Nothing in memstore to flush");
+    Status s = Status::IOError("Nothing in memstore to flush");
+    return s;
+  }
+
+  // record the logfile_number_ before we release the mutex
+  VersionEdit* edit = m->GetEdits();
+  edit->SetPrevLogNumber(0);
+  edit->SetLogNumber(logfile_number_);  // Earlier logs no longer needed
+
+  Status s = WriteLevel0Table(m, edit, &file_number);
+
   if (s.ok() && shutting_down_.Acquire_Load()) {
     s = Status::IOError("Deleting DB during memtable compaction");
   }
 
   // Replace immutable memtable with the generated Table
-  if (s.ok()) {
-    edit.SetPrevLogNumber(0);
-    edit.SetLogNumber(logfile_number_);  // Earlier logs no longer needed
-    s = versions_->LogAndApply(&edit, &mutex_);
-  }
+  s = imm_.InstallMemtableFlushResults(m, versions_, s, &mutex_, 
+                        options_.info_log, file_number, pending_outputs_);
 
   if (s.ok()) {
-    // Commit to the new state
-    imm_->Unref();
-    imm_ = NULL;
-    has_imm_.Release_Store(NULL);
+    if (madeProgress) {
+      *madeProgress = 1;
+    }
     DeleteObsoleteFiles();
     MaybeScheduleLogDBDeployStats();
   }
-
   return s;
 }
 
@@ -636,6 +709,7 @@ void DBImpl::TEST_CompactRange(int level, const Slice* begin,const Slice* end) {
   ManualCompaction manual;
   manual.level = level;
   manual.done = false;
+  manual.in_progress = false;
   if (begin == NULL) {
     manual.begin = NULL;
   } else {
@@ -650,16 +724,46 @@ void DBImpl::TEST_CompactRange(int level, const Slice* begin,const Slice* end) {
   }
 
   MutexLock l(&mutex_);
+
+  // When a manual compaction arrives, temporarily throttle down 
+  // the number of background compaction threads to 1. This is 
+  // needed to ensure that this manual compaction can compact 
+  // any range of keys/files. We artificialy increase 
+  // bg_compaction_scheduled_ by a large number, this causes
+  // the system to have a single background thread. Now,
+  // this manual compaction can progress without stomping
+  // on any other concurrent compactions.
+  const int LargeNumber = 10000000;
+  const int newvalue = options_.max_background_compactions-1;
+  bg_compaction_scheduled_ += LargeNumber;
+  while (bg_compaction_scheduled_ > LargeNumber) {
+    Log(options_.info_log, "Manual compaction request waiting for background threads to fall below 1");
+    bg_cv_.Wait();
+  }
+  Log(options_.info_log, "Manual compaction starting");
+
   while (!manual.done) {
     while (manual_compaction_ != NULL) {
       bg_cv_.Wait();
     }
     manual_compaction_ = &manual;
+    if (bg_compaction_scheduled_ == LargeNumber) {
+      bg_compaction_scheduled_ = newvalue;
+    }
     MaybeScheduleCompaction();
     while (manual_compaction_ == &manual) {
       bg_cv_.Wait();
     }
   }
+  assert(!manual.in_progress);
+
+  // wait till there are no background threads scheduled
+  bg_compaction_scheduled_ += LargeNumber;
+  while (bg_compaction_scheduled_ > LargeNumber + newvalue) {
+    Log(options_.info_log, "Manual compaction resetting background threads");
+    bg_cv_.Wait();
+  }
+  bg_compaction_scheduled_ = 0;
 }
 
 Status DBImpl::FlushMemTable(const FlushOptions& options) {
@@ -676,10 +780,10 @@ Status DBImpl::WaitForCompactMemTable() {
   Status s;
   // Wait until the compaction completes
   MutexLock l(&mutex_);
-  while (imm_ != NULL && bg_error_.ok()) {
+  while (imm_.size() > 0 && bg_error_.ok()) {
     bg_cv_.Wait();
   }
-  if (imm_ != NULL) {
+  if (imm_.size() != 0) {
     s = bg_error_;
   }
   return s;
@@ -704,16 +808,16 @@ Status DBImpl::TEST_WaitForCompact() {
 
 void DBImpl::MaybeScheduleCompaction() {
   mutex_.AssertHeld();
-  if (bg_compaction_scheduled_) {
+  if (bg_compaction_scheduled_ >= options_.max_background_compactions) {
     // Already scheduled
   } else if (shutting_down_.Acquire_Load()) {
     // DB is being deleted; no more background compactions
-  } else if (imm_ == NULL &&
+  } else if (!imm_.IsFlushPending() &&
              manual_compaction_ == NULL &&
              !versions_->NeedsCompaction()) {
     // No work to be done
   } else {
-    bg_compaction_scheduled_ = true;
+    bg_compaction_scheduled_++;
     env_->Schedule(&DBImpl::BGWork, this);
   }
 }
@@ -723,10 +827,12 @@ void DBImpl::BGWork(void* db) {
 }
 
 void DBImpl::BackgroundCall() {
+  bool madeProgress;
   MutexLock l(&mutex_);
+  // Log(options_.info_log, "XXX BG Thread %llx process new work item", pthread_self());
   assert(bg_compaction_scheduled_);
   if (!shutting_down_.Acquire_Load()) {
-    Status s = BackgroundCompaction();
+    Status s = BackgroundCompaction(&madeProgress);
     if (!s.ok()) {
       // Wait a little bit before retrying background compaction in
       // case this is an environmental problem and we do not want to
@@ -741,28 +847,41 @@ void DBImpl::BackgroundCall() {
     }
   }
 
-  bg_compaction_scheduled_ = false;
+  bg_compaction_scheduled_--;
 
   MaybeScheduleLogDBDeployStats();
 
   // Previous compaction may have produced too many files in a level,
-  // so reschedule another compaction if needed.
-  MaybeScheduleCompaction();
+  // So reschedule another compaction if we made progress in the 
+  // last compaction.
+  if (madeProgress) {
+    MaybeScheduleCompaction();
+  }
   bg_cv_.SignalAll();
 }
 
-Status DBImpl::BackgroundCompaction() {
+Status DBImpl::BackgroundCompaction(bool* madeProgress) {
+  *madeProgress = false;
   mutex_.AssertHeld();
 
-  if (imm_ != NULL) {
-    return CompactMemTable();
+  while (imm_.IsFlushPending()) {
+    Log(options_.info_log, 
+        "BackgroundCompaction doing CompactMemTable, compaction slots available %d",
+        options_.max_background_compactions - bg_compaction_scheduled_);
+    Status stat = CompactMemTable(madeProgress);
+    if (!stat.ok()) {
+      return stat;
+    }
   }
 
   Compaction* c;
-  bool is_manual = (manual_compaction_ != NULL);
+  bool is_manual = (manual_compaction_ != NULL) &&
+                   (manual_compaction_->in_progress == false);
   InternalKey manual_end;
   if (is_manual) {
     ManualCompaction* m = manual_compaction_;
+    assert(!m->in_progress);
+    m->in_progress = true; // another thread cannot pick up the same work
     c = versions_->CompactRange(m->level, m->begin, m->end);
     m->done = (c == NULL);
     if (c != NULL) {
@@ -781,6 +900,7 @@ Status DBImpl::BackgroundCompaction() {
   Status status;
   if (c == NULL) {
     // Nothing to do
+    Log(options_.info_log, "Compaction nothing to do");
   } else if (!is_manual && c->IsTrivialMove()) {
     // Move file to next level
     assert(c->num_input_files(0) == 1);
@@ -796,12 +916,16 @@ Status DBImpl::BackgroundCompaction() {
         static_cast<unsigned long long>(f->file_size),
         status.ToString().c_str(),
         versions_->LevelSummary(&tmp));
+    versions_->ReleaseCompactionFiles(c);
+    *madeProgress = true;
   } else {
     CompactionState* compact = new CompactionState(c);
     status = DoCompactionWork(compact);
     CleanupCompaction(compact);
+    versions_->ReleaseCompactionFiles(c);
     c->ReleaseInputs();
     DeleteObsoleteFiles();
+    *madeProgress = true;
   }
   delete c;
 
@@ -828,6 +952,7 @@ Status DBImpl::BackgroundCompaction() {
       m->tmp_storage = manual_end;
       m->begin = &m->tmp_storage;
     }
+    m->in_progress = false; // not being processed anymore
     manual_compaction_ = NULL;
   }
   return status;
@@ -850,21 +975,54 @@ void DBImpl::CleanupCompaction(CompactionState* compact) {
   delete compact;
 }
 
+// Allocate the file numbers for the output file. We allocate as
+// many output file numbers as there are files in level+1.
+// Insert them into pending_outputs so that they do not get deleted.
+void DBImpl::AllocateCompactionOutputFileNumbers(CompactionState* compact) {
+  mutex_.AssertHeld();
+  assert(compact != NULL);
+  assert(compact->builder == NULL);
+  int filesNeeded = compact->compaction->num_input_files(1);
+  for (unsigned i = 0; i < filesNeeded; i++) {
+    uint64_t file_number = versions_->NewFileNumber();
+    pending_outputs_.insert(file_number);
+    compact->allocated_file_numbers.push_back(file_number);
+  }
+}
+
+// Frees up unused file number.
+void DBImpl::ReleaseCompactionUnusedFileNumbers(CompactionState* compact) {
+  mutex_.AssertHeld();
+  for (std::list<uint64_t>::iterator it = 
+       compact->allocated_file_numbers.begin();
+       it != compact->allocated_file_numbers.end(); ++it) {
+    uint64_t file_number = *it;
+    pending_outputs_.erase(file_number);
+    // Log(options_.info_log, "XXX releasing unused file num %d", file_number);
+  }
+}
+
 Status DBImpl::OpenCompactionOutputFile(CompactionState* compact) {
   assert(compact != NULL);
   assert(compact->builder == NULL);
   uint64_t file_number;
-  {
+  // If we have not yet exhausted the pre-allocated file numbers,
+  // then use the one from the front. Otherwise, we have to acquire
+  // the heavyweight lock and allocate a new file number.
+  if (!compact->allocated_file_numbers.empty()) {
+    file_number = compact->allocated_file_numbers.front();
+    compact->allocated_file_numbers.pop_front();
+  } else {
     mutex_.Lock();
     file_number = versions_->NewFileNumber();
     pending_outputs_.insert(file_number);
-    CompactionState::Output out;
-    out.number = file_number;
-    out.smallest.Clear();
-    out.largest.Clear();
-    compact->outputs.push_back(out);
     mutex_.Unlock();
   }
+  CompactionState::Output out;
+  out.number = file_number;
+  out.smallest.Clear();
+  out.largest.Clear();
+  compact->outputs.push_back(out);
 
   // Make the output file
   std::string fname = TableFileName(dbname_, file_number);
@@ -933,6 +1091,21 @@ Status DBImpl::FinishCompactionOutputFile(CompactionState* compact,
 
 Status DBImpl::InstallCompactionResults(CompactionState* compact) {
   mutex_.AssertHeld();
+
+  // paranoia: verify that the files that we started with
+  // still exist in the current version and in the same original level.
+  // This ensures that a concurrent compaction did not erroneously
+  // pick the same files to compact.
+  if (options_.paranoid_checks &&
+      !versions_->VerifyCompactionFileConsistency(compact->compaction)) {
+    Log(options_.info_log,  "Compaction %d@%d + %d@%d files aborted",
+      compact->compaction->num_input_files(0),
+      compact->compaction->level(),
+      compact->compaction->num_input_files(1),
+      compact->compaction->level() + 1);
+    return Status::IOError("Compaction input files inconsistent");
+  }
+
   Log(options_.info_log,  "Compacted %d@%d + %d@%d files => %lld bytes",
       compact->compaction->num_input_files(0),
       compact->compaction->level(),
@@ -953,14 +1126,15 @@ Status DBImpl::InstallCompactionResults(CompactionState* compact) {
 }
 
 Status DBImpl::DoCompactionWork(CompactionState* compact) {
-  const uint64_t start_micros = env_->NowMicros();
   int64_t imm_micros = 0;  // Micros spent doing imm_ compactions
 
-  Log(options_.info_log,  "Compacting %d@%d + %d@%d files",
+  Log(options_.info_log, 
+      "Compacting %d@%d + %d@%d files, compaction slots available %d",
       compact->compaction->num_input_files(0),
       compact->compaction->level(),
       compact->compaction->num_input_files(1),
-      compact->compaction->level() + 1);
+      compact->compaction->level() + 1,
+      options_.max_background_compactions - bg_compaction_scheduled_);
   char scratch[256];
   compact->compaction->Summary(scratch, sizeof(scratch));
   Log(options_.info_log, "Compaction start summary: %s\n", scratch);
@@ -974,9 +1148,13 @@ Status DBImpl::DoCompactionWork(CompactionState* compact) {
     compact->smallest_snapshot = snapshots_.oldest()->number_;
   }
 
+  // Allocate the output file numbers before we release the lock
+  AllocateCompactionOutputFileNumbers(compact);
+
   // Release mutex while we're actually doing the compaction work
   mutex_.Unlock();
 
+  const uint64_t start_micros = env_->NowMicros();
   Iterator* input = versions_->MakeInputIterator(compact->compaction);
   input->SeekToFirst();
   Status status;
@@ -986,10 +1164,10 @@ Status DBImpl::DoCompactionWork(CompactionState* compact) {
   SequenceNumber last_sequence_for_key = kMaxSequenceNumber;
   for (; input->Valid() && !shutting_down_.Acquire_Load(); ) {
     // Prioritize immutable compaction work
-    if (has_imm_.NoBarrier_Load() != NULL) {
+    if (imm_.imm_flush_needed.NoBarrier_Load() != NULL) {
       const uint64_t imm_start = env_->NowMicros();
       mutex_.Lock();
-      if (imm_ != NULL) {
+      if (imm_.IsFlushPending()) {
         CompactMemTable();
         bg_cv_.SignalAll();  // Wakeup MakeRoomForWrite() if necessary
       }
@@ -1101,15 +1279,23 @@ Status DBImpl::DoCompactionWork(CompactionState* compact) {
     stats.bytes_written += compact->outputs[i].file_size;
   }
 
+  int MBpersec = ((stats.bytes_read + stats.bytes_written))
+                                     /stats.micros;
+
   mutex_.Lock();
   stats_[compact->compaction->level() + 1].Add(stats);
 
+  // if there were any unused file number (mostly in case of
+  // compaction error), free up the entry from pending_putputs
+  ReleaseCompactionUnusedFileNumbers(compact);
+
   if (status.ok()) {
     status = InstallCompactionResults(compact);
   }
   VersionSet::LevelSummaryStorage tmp;
   Log(options_.info_log,
-      "compacted to: %s", versions_->LevelSummary(&tmp));
+      "compacted to: %s %d MBytes/sec", versions_->LevelSummary(&tmp),
+                                       MBpersec);
   return status;
 }
 
@@ -1117,15 +1303,15 @@ namespace {
 struct IterState {
   port::Mutex* mu;
   Version* version;
-  MemTable* mem;
-  MemTable* imm;
+  std::vector<MemTable*> mem; // includes both mem_ and imm_
 };
 
 static void CleanupIteratorState(void* arg1, void* arg2) {
   IterState* state = reinterpret_cast<IterState*>(arg1);
   state->mu->Lock();
-  state->mem->Unref();
-  if (state->imm != NULL) state->imm->Unref();
+  for (unsigned int i = 0; i < state->mem.size(); i++) {
+    state->mem[i]->Unref();
+  }
   state->version->Unref();
   state->mu->Unlock();
   delete state;
@@ -1138,22 +1324,29 @@ Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
   mutex_.Lock();
   *latest_snapshot = versions_->LastSequence();
 
-  // Collect together all needed child iterators
+  // Collect together all needed child iterators for mem
   std::vector<Iterator*> list;
-  list.push_back(mem_->NewIterator());
   mem_->Ref();
-  if (imm_ != NULL) {
-    list.push_back(imm_->NewIterator());
-    imm_->Ref();
+  list.push_back(mem_->NewIterator());
+  cleanup->mem.push_back(mem_);
+
+  // Collect together all needed child iterators for imm_
+  std::vector<MemTable*> immutables;
+  imm_.GetMemTables(&immutables);
+  for (unsigned int i = 0; i < immutables.size(); i++) {
+    MemTable* m = immutables[i];
+    m->Ref();
+    list.push_back(m->NewIterator());
+    cleanup->mem.push_back(m);
   }
+
+  // Collect iterators for files in L0 - Ln
   versions_->current()->AddIterators(options, &list);
   Iterator* internal_iter =
       NewMergingIterator(&internal_comparator_, &list[0], list.size());
   versions_->current()->Ref();
 
   cleanup->mu = &mutex_;
-  cleanup->mem = mem_;
-  cleanup->imm = imm_;
   cleanup->version = versions_->current();
   internal_iter->RegisterCleanup(CleanupIteratorState, cleanup, NULL);
 
@@ -1184,10 +1377,10 @@ Status DBImpl::Get(const ReadOptions& options,
   }
 
   MemTable* mem = mem_;
-  MemTable* imm = imm_;
+  MemTableList imm = imm_;
   Version* current = versions_->current();
   mem->Ref();
-  if (imm != NULL) imm->Ref();
+  imm.RefAll();
   current->Ref();
 
   bool have_stat_update = false;
@@ -1200,7 +1393,7 @@ Status DBImpl::Get(const ReadOptions& options,
     LookupKey lkey(key, snapshot);
     if (mem->Get(lkey, value, &s)) {
       // Done
-    } else if (imm != NULL && imm->Get(lkey, value, &s)) {
+    } else if (imm.Get(lkey, value, &s)) {
       // Done
     } else {
       s = current->Get(options, lkey, value, &stats);
@@ -1214,7 +1407,7 @@ Status DBImpl::Get(const ReadOptions& options,
     MaybeScheduleCompaction();
   }
   mem->Unref();
-  if (imm != NULL) imm->Unref();
+  imm.UnrefAll();
   current->Unref();
   return s;
 }
@@ -1399,24 +1592,30 @@ Status DBImpl::MakeRoomForWrite(bool force) {
       mutex_.Unlock();
       env_->SleepForMicroseconds(1000);
       allow_delay = false;  // Do not delay a single write more than once
-      Log(options_.info_log, "delaying write...\n");
       mutex_.Lock();
+      delayed_writes_++;
     } else if (!force &&
                (mem_->ApproximateMemoryUsage() <= options_.write_buffer_size)) {
       // There is room in current memtable
+      if (allow_delay) {
+        DelayLoggingAndReset();
+      }
       break;
-    } else if (imm_ != NULL) {
+    } else if (imm_.size() == options_.max_write_buffer_number - 1) {
       // We have filled up the current memtable, but the previous
-      // one is still being compacted, so we wait.
+      // ones are still being compacted, so we wait.
+      DelayLoggingAndReset();
       Log(options_.info_log, "wait for memtable compaction...\n");
       bg_cv_.Wait();
     } else if (versions_->NumLevelFiles(0) >=
 		options_.level0_stop_writes_trigger) {
       // There are too many level-0 files.
-      Log(options_.info_log, "waiting...\n");
+      DelayLoggingAndReset();
+      Log(options_.info_log, "wait for fewer level0 files...\n");
       bg_cv_.Wait();
     } else {
       // Attempt to switch to a new memtable and trigger compaction of old
+      DelayLoggingAndReset();
       assert(versions_->PrevLogNumber() == 0);
       uint64_t new_log_number = versions_->NewFileNumber();
       WritableFile* lfile = NULL;
@@ -1431,9 +1630,8 @@ Status DBImpl::MakeRoomForWrite(bool force) {
       logfile_ = lfile;
       logfile_number_ = new_log_number;
       log_ = new log::Writer(lfile);
-      imm_ = mem_;
-      has_imm_.Release_Store(imm_);
-      mem_ = new MemTable(internal_comparator_);
+      imm_.Add(mem_);
+      mem_ = new MemTable(internal_comparator_, NumberLevels());
       mem_->Ref();
       force = false;   // Do not force another compaction if have room
       MaybeScheduleCompaction();
@@ -1522,6 +1720,13 @@ void DBImpl::GetApproximateSizes(
   }
 }
 
+inline void DBImpl::DelayLoggingAndReset() {
+  if (delayed_writes_ > 0) {
+    Log(options_.info_log, "delayed %d write...\n", delayed_writes_ );
+    delayed_writes_ = 0;
+  }
+}
+
 // Default implementations of convenience methods that subclasses of DB
 // can call if they wish
 Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {
diff --git a/db/db_impl.h b/db/db_impl.h
index e08025eca..a011dea7d 100644
--- a/db/db_impl.h
+++ b/db/db_impl.h
@@ -14,6 +14,7 @@
 #include "leveldb/env.h"
 #include "port/port.h"
 #include "util/stats_logger.h"
+#include "memtablelist.h"
 
 #ifdef USE_SCRIBE
 #include "scribe/scribe_logger.h"
@@ -99,13 +100,20 @@ class DBImpl : public DB {
 
   // Compact the in-memory write buffer to disk.  Switches to a new
   // log-file/memtable and writes a new descriptor iff successful.
-  Status CompactMemTable();
+  Status CompactMemTable(bool* madeProgress = NULL);
 
   Status RecoverLogFile(uint64_t log_number,
                         VersionEdit* edit,
                         SequenceNumber* max_sequence);
 
-  Status WriteLevel0Table(MemTable* mem, VersionEdit* edit, Version* base);
+  // The following two methods are used to flush a memtable to
+  // storage. The first one is used atdatabase RecoveryTime (when the
+  // database is opened) and is heavyweight because it holds the mutex
+  // for the entire period. The second method WriteLevel0Table supports
+  // concurrent flush memtables to storage.
+  Status WriteLevel0TableForRecovery(MemTable* mem, VersionEdit* edit);
+  Status WriteLevel0Table(MemTable* mem, VersionEdit* edit,
+                                uint64_t* filenumber);
 
   Status MakeRoomForWrite(bool force /* compact even if there is room? */);
   WriteBatch* BuildBatchGroup(Writer** last_writer);
@@ -123,13 +131,16 @@ class DBImpl : public DB {
   void MaybeScheduleCompaction();
   static void BGWork(void* db);
   void BackgroundCall();
-  Status BackgroundCompaction();
+  Status BackgroundCompaction(bool* madeProgress);
   void CleanupCompaction(CompactionState* compact);
   Status DoCompactionWork(CompactionState* compact);
 
   Status OpenCompactionOutputFile(CompactionState* compact);
   Status FinishCompactionOutputFile(CompactionState* compact, Iterator* input);
   Status InstallCompactionResults(CompactionState* compact);
+  void AllocateCompactionOutputFileNumbers(CompactionState* compact);
+  void ReleaseCompactionUnusedFileNumbers(CompactionState* compact);
+ 
 
   // Constant after construction
   Env* const env_;
@@ -151,8 +162,7 @@ class DBImpl : public DB {
   port::AtomicPointer shutting_down_;
   port::CondVar bg_cv_;          // Signalled when background work finishes
   MemTable* mem_;
-  MemTable* imm_;                // Memtable being compacted
-  port::AtomicPointer has_imm_;  // So bg thread can detect non-NULL imm_
+  MemTableList imm_;             // Memtable that are not changing
   WritableFile* logfile_;
   uint64_t logfile_number_;
   log::Writer* log_;
@@ -169,8 +179,8 @@ class DBImpl : public DB {
   // part of ongoing compactions.
   std::set<uint64_t> pending_outputs_;
 
-  // Has a background compaction been scheduled or is running?
-  bool bg_compaction_scheduled_;
+  // count how many background compaction been scheduled or is running?
+  int bg_compaction_scheduled_;
 
   // Has a background stats log thread scheduled?
   bool bg_logstats_scheduled_;
@@ -179,6 +189,7 @@ class DBImpl : public DB {
   struct ManualCompaction {
     int level;
     bool done;
+    bool in_progress;           // compaction request being processed?
     const InternalKey* begin;   // NULL means beginning of key range
     const InternalKey* end;     // NULL means end of key range
     InternalKey tmp_storage;    // Used to keep track of compaction progress
@@ -220,6 +231,9 @@ class DBImpl : public DB {
   static const int KEEP_LOG_FILE_NUM = 1000;
   std::string db_absolute_path_;
 
+  // count of the number of contiguous delaying writes
+  int delayed_writes_;
+
   // No copying allowed
   DBImpl(const DBImpl&);
   void operator=(const DBImpl&);
@@ -227,6 +241,9 @@ class DBImpl : public DB {
   const Comparator* user_comparator() const {
     return internal_comparator_.user_comparator();
   }
+
+  // dump the delayed_writes_ to the log file and reset counter.
+  void DelayLoggingAndReset();
 };
 
 // Sanitize db options.  The caller should delete result.info_log if
diff --git a/db/memtable.cc b/db/memtable.cc
index bfec0a7e7..cbd94919a 100644
--- a/db/memtable.cc
+++ b/db/memtable.cc
@@ -18,10 +18,14 @@ static Slice GetLengthPrefixedSlice(const char* data) {
   return Slice(p, len);
 }
 
-MemTable::MemTable(const InternalKeyComparator& cmp)
+MemTable::MemTable(const InternalKeyComparator& cmp, int numlevel)
     : comparator_(cmp),
       refs_(0),
-      table_(comparator_, &arena_) {
+      table_(comparator_, &arena_),
+      flush_in_progress_(false),
+      flush_completed_(false),
+      file_number_(0), 
+      edit_(numlevel) {
 }
 
 MemTable::~MemTable() {
@@ -101,7 +105,7 @@ void MemTable::Add(SequenceNumber s, ValueType type,
   p += 8;
   p = EncodeVarint32(p, val_size);
   memcpy(p, value.data(), val_size);
-  assert((p + val_size) - buf == encoded_len);
+  assert((p + val_size) - buf == (unsigned)encoded_len);
   table_.Insert(buf);
 }
 
diff --git a/db/memtable.h b/db/memtable.h
index 92e90bb09..fa918c922 100644
--- a/db/memtable.h
+++ b/db/memtable.h
@@ -9,6 +9,7 @@
 #include "leveldb/db.h"
 #include "db/dbformat.h"
 #include "db/skiplist.h"
+#include "db/version_set.h"
 #include "util/arena.h"
 
 namespace leveldb {
@@ -21,7 +22,8 @@ class MemTable {
  public:
   // MemTables are reference counted.  The initial reference count
   // is zero and the caller must call Ref() at least once.
-  explicit MemTable(const InternalKeyComparator& comparator);
+  explicit MemTable(const InternalKeyComparator& comparator,
+                    int numlevel = 7);
 
   // Increase reference count.
   void Ref() { ++refs_; }
@@ -63,6 +65,9 @@ class MemTable {
   // Else, return false.
   bool Get(const LookupKey& key, std::string* value, Status* s);
 
+  // Returns the edits area that is needed for flushing the memtable
+  VersionEdit* GetEdits() { return &edit_; }
+
  private:
   ~MemTable();  // Private since only Unref() should be used to delete it
 
@@ -73,6 +78,7 @@ class MemTable {
   };
   friend class MemTableIterator;
   friend class MemTableBackwardIterator;
+  friend class MemTableList;
 
   typedef SkipList<const char*, KeyComparator> Table;
 
@@ -81,6 +87,15 @@ class MemTable {
   Arena arena_;
   Table table_;
 
+  // These are used to manage memtable flushes to storage
+  bool flush_in_progress_; // started the flush 
+  bool flush_completed_;   // finished the flush
+  uint64_t file_number_;    // filled up after flush is complete
+
+  // The udpates to be applied to the transaction log when this
+  // memtable is flushed to storage.
+  VersionEdit edit_;
+
   // No copying allowed
   MemTable(const MemTable&);
   void operator=(const MemTable&);
diff --git a/db/memtablelist.cc b/db/memtablelist.cc
new file mode 100644
index 000000000..916dd902f
--- /dev/null
+++ b/db/memtablelist.cc
@@ -0,0 +1,192 @@
+// Copyright (c) 2012 Facebook.
+
+#include <string>
+#include "leveldb/db.h"
+#include "db/memtable.h"
+#include "db/memtablelist.h"
+#include "leveldb/env.h"
+#include "leveldb/iterator.h"
+#include "util/coding.h"
+
+namespace leveldb {
+
+class InternalKeyComparator;
+class Mutex;
+class MemTableListIterator;
+class VersionSet;
+
+using std::list;
+
+// Increase reference count on all underling memtables
+void MemTableList::RefAll() {
+  for (list<MemTable*>::iterator it = memlist_.begin();
+       it != memlist_.end() ; ++it) {
+    (*it)->Ref();
+  }
+}
+
+// Drop reference count on all underling memtables
+void MemTableList::UnrefAll() {
+  for (list<MemTable*>::iterator it = memlist_.begin();
+       it != memlist_.end() ; ++it) {
+    (*it)->Unref();
+  }
+}
+
+// Returns the total number of memtables in the list
+int MemTableList::size() {
+  assert(num_flush_not_started_ <= size_);
+  return size_;
+}
+
+// Returns true if there is at least one memtable on which flush has
+// not yet started.
+bool MemTableList::IsFlushPending() {
+  if (num_flush_not_started_ > 0) {
+    assert(imm_flush_needed.NoBarrier_Load() != NULL);
+    return true;
+  }
+  return false;
+}
+
+// Returns the earliest memtable that needs to be flushed.
+// Returns null, if no such memtable exist.
+MemTable* MemTableList::PickMemtableToFlush() {
+  for (list<MemTable*>::reverse_iterator it = memlist_.rbegin();
+       it != memlist_.rend(); it++) {
+    MemTable* m = *it;
+    if (!m->flush_in_progress_) {
+      assert(!m->flush_completed_);
+      num_flush_not_started_--;    
+      if (num_flush_not_started_ == 0) {
+        imm_flush_needed.Release_Store(NULL);
+      }
+      m->flush_in_progress_ = true; // flushing will start very soon
+      return m;
+    }
+  }
+  return NULL;
+}
+
+// Record a successful flush in the manifest file
+Status MemTableList::InstallMemtableFlushResults(MemTable* m,
+                      VersionSet* vset, Status flushStatus,
+                      port::Mutex* mu, Logger* info_log,
+                      uint64_t file_number,
+                      std::set<uint64_t>& pending_outputs) {
+  mu->AssertHeld();
+  assert(m->flush_in_progress_);
+  assert(m->file_number_ == 0);
+
+  // If the flush was not successful, then just reset state.
+  // Maybe a suceeding attempt to flush will be successful.
+  if (!flushStatus.ok()) {
+    m->flush_in_progress_ = false;
+    m->flush_completed_ = false;
+    m->edit_.Clear();
+    num_flush_not_started_++;
+    imm_flush_needed.Release_Store((void *)1);
+    pending_outputs.erase(file_number);
+    return flushStatus;
+  }
+
+  // flush was sucessful
+  m->flush_completed_ = true;
+  m->file_number_ = file_number;
+
+  // if some other thread is already commiting, then return
+  Status s;
+  if (commit_in_progress_) {
+    return s;
+  }
+
+  // Only a single thread can be executing this piece of code
+  commit_in_progress_ = true;
+
+  // scan all memtables from the earliest, and commit those
+  // (in that order) that have finished flushing.
+  while (!memlist_.empty()) {
+    m = memlist_.back(); // get the last element
+    if (!m->flush_completed_) {
+      break;
+    }
+    Log(info_log, 
+        "Level-0 commit table #%llu: started",
+        (unsigned long long)m->file_number_);
+
+    // this can release and reacquire the mutex. 
+    s = vset->LogAndApply(&m->edit_, mu);
+
+    if (s.ok()) { // commit new state
+      Log(info_log, "Level-0 commit table #%llu: done",
+                     (unsigned long long)m->file_number_);
+      memlist_.remove(m);
+      assert(m->file_number_ > 0);
+
+      // pending_outputs can be cleared only after the newly created file
+      // has been written to a committed version so that other concurrently
+      // executing compaction threads do not mistakenly assume that this
+      // file is not live.
+      pending_outputs.erase(m->file_number_);
+      m->Unref();
+      size_--;
+    } else {
+      //commit failed. setup state so that we can flush again.
+      Log(info_log, "Level-0 commit table #%llu: failed",
+                     (unsigned long long)m->file_number_);
+      m->flush_completed_ = false;
+      m->flush_in_progress_ = false;
+      m->edit_.Clear();
+      num_flush_not_started_++;
+      pending_outputs.erase(m->file_number_);
+      m->file_number_ = 0;
+      imm_flush_needed.Release_Store((void *)1);
+      s = Status::IOError("Unable to commit flushed memtable");
+      break;
+    }
+  }
+  commit_in_progress_ = false;
+  return s;
+}
+
+// New memtables are inserted at the front of the list. 
+void MemTableList::Add(MemTable* m) {
+  assert(size_ >= num_flush_not_started_);
+  size_++; 
+  memlist_.push_front(m); 
+  num_flush_not_started_++;
+  if (num_flush_not_started_ == 1) {
+    imm_flush_needed.Release_Store((void *)1);
+  }
+}
+
+// Returns an estimate of the number of bytes of data in use.
+size_t MemTableList::ApproximateMemoryUsage() {
+  size_t size = 0;
+  for (list<MemTable*>::iterator it = memlist_.begin();
+       it != memlist_.end(); ++it) {
+    size += (*it)->ApproximateMemoryUsage();
+  }
+  return size;
+}
+
+// Search all the memtables starting from the most recent one.
+// Return the most recent value found, if any.
+bool MemTableList::Get(const LookupKey& key, std::string* value, Status* s) {
+  for (list<MemTable*>::iterator it = memlist_.begin();
+       it != memlist_.end(); ++it) {
+    if ((*it)->Get(key, value, s)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+void MemTableList::GetMemTables(std::vector<MemTable*>* output) {
+  for (list<MemTable*>::iterator it = memlist_.begin();
+       it != memlist_.end(); ++it) {
+    output->push_back(*it);
+  }
+}
+
+}  // namespace leveldb
diff --git a/db/memtablelist.h b/db/memtablelist.h
new file mode 100644
index 000000000..76008a8c2
--- /dev/null
+++ b/db/memtablelist.h
@@ -0,0 +1,96 @@
+// Copyright (c) 2012 Facebook.
+
+#ifndef STORAGE_LEVELDB_DB_MEMTABLELIST_H_
+#define STORAGE_LEVELDB_DB_MEMTABLELIST_H_
+
+#include <string>
+#include <list>
+#include "leveldb/db.h"
+#include "db/dbformat.h"
+#include "db/skiplist.h"
+#include "util/arena.h"
+#include "memtable.h"
+
+namespace leveldb {
+
+class InternalKeyComparator;
+class Mutex;
+class MemTableListIterator;
+
+//
+// This class stores refeernces to all the immutable memtables.
+// The memtables are flushed to L0 as soon as possible and in 
+// any order. If there are more than one immutable memtable, their
+// flushes can occur concurrently.  However, they are 'committed' 
+// to the manifest in FIFO order to maintain correctness and
+// recoverability from a crash.
+//
+class MemTableList {
+ public:
+  // A list of memtables. 
+  MemTableList() : size_(0), num_flush_not_started_(0),
+    commit_in_progress_(false) {
+    imm_flush_needed.Release_Store(NULL);
+  }
+  ~MemTableList() {};
+
+  // so that backgrund threads can detect non-NULL pointer to
+  // determine whether this is anything more to start flushing.
+  port::AtomicPointer imm_flush_needed;
+
+  // Increase reference count on all underling memtables
+  void RefAll();
+
+  // Drop reference count on all underling memtables
+  void UnrefAll();
+
+  // Returns the total number of memtables in the list
+  int size();
+
+  // Returns true if there is at least one memtable on which flush has
+  // not yet started.
+  bool IsFlushPending();
+
+  // Returns the earliest memtable that needs to be flushed.
+  // Returns null, if no such memtable exist.
+  MemTable* PickMemtableToFlush();
+
+  // Commit a successful flush in the manifest file
+  Status InstallMemtableFlushResults(MemTable* m,
+                      VersionSet* vset, Status flushStatus,
+                      port::Mutex* mu, Logger* info_log,
+                      uint64_t file_number,
+                      std::set<uint64_t>& pending_outputs);
+
+  // New memtables are inserted at the front of the list. 
+  void Add(MemTable* m);
+
+  // Returns an estimate of the number of bytes of data in use.
+  size_t ApproximateMemoryUsage();
+
+  // Search all the memtables starting from the most recent one.
+  // Return the most recent value found, if any.
+  bool Get(const LookupKey& key, std::string* value, Status* s);
+
+  // Returns the list of underlying memtables. 
+  void GetMemTables(std::vector<MemTable*>* list);
+
+  // Copying allowed
+  // MemTableList(const MemTableList&);
+  // void operator=(const MemTableList&);
+
+ private:
+  std::list<MemTable*> memlist_;
+  int size_;
+
+  // the number of elements that still need flushing
+  int num_flush_not_started_;
+
+  // committing in progress
+  bool commit_in_progress_;
+
+};
+
+}  // namespace leveldb
+
+#endif  // STORAGE_LEVELDB_DB_MEMTABLELIST_H_
diff --git a/db/repair.cc b/db/repair.cc
index 3a2d038c8..4b0220b5c 100644
--- a/db/repair.cc
+++ b/db/repair.cc
@@ -199,7 +199,7 @@ class Repairer {
     std::string scratch;
     Slice record;
     WriteBatch batch;
-    MemTable* mem = new MemTable(icmp_);
+    MemTable* mem = new MemTable(icmp_, options_.num_levels);
     mem->Ref();
     int counter = 0;
     while (reader.ReadRecord(&record, &scratch)) {
diff --git a/db/skiplist.h b/db/skiplist.h
index ba62641c8..9cb67bbe6 100644
--- a/db/skiplist.h
+++ b/db/skiplist.h
@@ -23,6 +23,10 @@
 // more lists.
 //
 // ... prev vs. next pointer ordering ...
+//
+
+#ifndef STORAGE_LEVELDB_DB_SKIPLIST_H_
+#define STORAGE_LEVELDB_DB_SKIPLIST_H_
 
 #include <assert.h>
 #include <stdlib.h>
@@ -390,3 +394,5 @@ bool SkipList<Key,Comparator>::Contains(const Key& key) const {
 }
 
 }  // namespace leveldb
+
+#endif // STORAGE_LEVELDB_DB_SKIPLIST_H_
diff --git a/db/version_edit.h b/db/version_edit.h
index 320361754..159606448 100644
--- a/db/version_edit.h
+++ b/db/version_edit.h
@@ -21,8 +21,10 @@ struct FileMetaData {
   uint64_t file_size;         // File size in bytes
   InternalKey smallest;       // Smallest internal key served by table
   InternalKey largest;        // Largest internal key served by table
+  bool being_compacted;       // Is this file undergoing compaction?
 
-  FileMetaData() : refs(0), allowed_seeks(1 << 30), file_size(0) { }
+  FileMetaData() : refs(0), allowed_seeks(1 << 30), file_size(0),
+                   being_compacted(false) { }
 };
 
 class VersionEdit {
diff --git a/db/version_set.cc b/db/version_set.cc
index b00dc7737..d36438251 100644
--- a/db/version_set.cc
+++ b/db/version_set.cc
@@ -266,13 +266,14 @@ static bool NewestFirst(FileMetaData* a, FileMetaData* b) {
   return a->number > b->number;
 }
 
-Version::Version(VersionSet* vset)
+Version::Version(VersionSet* vset, uint64_t version_number)
     : vset_(vset), next_(this), prev_(this), refs_(0),
       file_to_compact_(NULL),
       file_to_compact_level_(-1),
-      compaction_score_(-1),
-      compaction_level_(-1),
-      offset_manifest_file_(0) {
+      compaction_score_(vset->NumberLevels()),
+      compaction_level_(vset->NumberLevels()),
+      offset_manifest_file_(0),
+      version_number_(version_number) {
   files_ = new std::vector<FileMetaData*>[vset->NumberLevels()];
 }
 
@@ -503,6 +504,8 @@ std::string Version::DebugString() const {
     //   20:43['e' .. 'g']
     r.append("--- level ");
     AppendNumberTo(&r, level);
+    r.append(" --- version# ");
+    AppendNumberTo(&r, version_number_);
     r.append(" ---\n");
     const std::vector<FileMetaData*>& files = files_[level];
     for (size_t i = 0; i < files.size(); i++) {
@@ -520,6 +523,17 @@ std::string Version::DebugString() const {
   return r;
 }
 
+// this is used to batch writes to the manifest file
+struct VersionSet::ManifestWriter {
+  Status status;
+  bool done;
+  port::CondVar cv;
+  VersionEdit* edit;
+  
+  explicit ManifestWriter(port::Mutex* mu, VersionEdit* e) : 
+             done(false), cv(mu), edit(e) {}
+};
+
 // A helper class so we can efficiently apply a whole sequence
 // of edits to a particular state without creating intermediate
 // Versions that contain full copies of the intermediate state.
@@ -586,8 +600,30 @@ class VersionSet::Builder {
     base_->Unref();
   }
 
+  void CheckConsistency(Version* v) {
+#ifndef NDEBUG
+    for (int level = 0; level < vset_->NumberLevels(); level++) {
+      // Make sure there is no overlap in levels > 0
+      if (level > 0) {
+        for (uint32_t i = 1; i < v->files_[level].size(); i++) {
+          const InternalKey& prev_end = v->files_[level][i-1]->largest;
+          const InternalKey& this_begin = v->files_[level][i]->smallest;
+          if (vset_->icmp_.Compare(prev_end, this_begin) >= 0) {
+            fprintf(stderr, "overlapping ranges in same level %s vs. %s\n",
+                    prev_end.DebugString().c_str(),
+                    this_begin.DebugString().c_str());
+            abort();
+          }
+        }
+      }
+    }
+#endif
+  }
+
   // Apply all of the edits in *edit to the current state.
   void Apply(VersionEdit* edit) {
+    CheckConsistency(base_);
+
     // Update compaction pointers
     for (size_t i = 0; i < edit->compact_pointers_.size(); i++) {
       const int level = edit->compact_pointers_[i].first;
@@ -603,6 +639,18 @@ class VersionSet::Builder {
       const int level = iter->first;
       const uint64_t number = iter->second;
       levels_[level].deleted_files.insert(number);
+
+#ifndef NDEBUG
+      // none of the files to be deleted could have been added
+      // by a concurrent compaction process
+      const FileSet* added = levels_[level].added_files;
+      for (FileSet::const_iterator added_iter = added->begin();
+         added_iter != added->end();
+         ++added_iter) {
+        FileMetaData* f = *added_iter;
+        assert(f->number != number);
+      }
+#endif
     }
 
     // Add new files
@@ -634,6 +682,8 @@ class VersionSet::Builder {
 
   // Save the current state in *v.
   void SaveTo(Version* v) {
+    CheckConsistency(base_);
+    CheckConsistency(v);
     BySmallestKey cmp;
     cmp.internal_comparator = &vset_->icmp_;
     for (int level = 0; level < vset_->NumberLevels(); level++) {
@@ -662,22 +712,7 @@ class VersionSet::Builder {
       for (; base_iter != base_end; ++base_iter) {
         MaybeAddFile(v, level, *base_iter);
       }
-
-#ifndef NDEBUG
-      // Make sure there is no overlap in levels > 0
-      if (level > 0) {
-        for (uint32_t i = 1; i < v->files_[level].size(); i++) {
-          const InternalKey& prev_end = v->files_[level][i-1]->largest;
-          const InternalKey& this_begin = v->files_[level][i]->smallest;
-          if (vset_->icmp_.Compare(prev_end, this_begin) >= 0) {
-            fprintf(stderr, "overlapping ranges in same level %s vs. %s\n",
-                    prev_end.DebugString().c_str(),
-                    this_begin.DebugString().c_str());
-            abort();
-          }
-        }
-      }
-#endif
+      CheckConsistency(v);
     }
   }
 
@@ -714,7 +749,9 @@ VersionSet::VersionSet(const std::string& dbname,
       descriptor_file_(NULL),
       descriptor_log_(NULL),
       dummy_versions_(this),
-      current_(NULL) {
+      current_(NULL),
+      compactions_in_progress_(options_->num_levels),
+      current_version_number_(0) {
   compact_pointer_ = new std::string[options_->num_levels];
   max_file_size_ = new uint64_t[options_->num_levels];
   level_max_bytes_ = new uint64_t[options->num_levels];
@@ -729,7 +766,7 @@ VersionSet::VersionSet(const std::string& dbname,
       level_max_bytes_[i] = options_->max_bytes_for_level_base;
     }
   }
-  AppendVersion(new Version(this));
+  AppendVersion(new Version(this, current_version_number_++));
 }
 
 VersionSet::~VersionSet() {
@@ -747,6 +784,7 @@ void VersionSet::AppendVersion(Version* v) {
   assert(v->refs_ == 0);
   assert(v != current_);
   if (current_ != NULL) {
+    assert(current_->refs_ > 0);
     current_->Unref();
   }
   current_ = v;
@@ -760,27 +798,35 @@ void VersionSet::AppendVersion(Version* v) {
 }
 
 Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
-  if (edit->has_log_number_) {
-    assert(edit->log_number_ >= log_number_);
-    assert(edit->log_number_ < next_file_number_);
-  } else {
-    edit->SetLogNumber(log_number_);
-  }
+  mu->AssertHeld();
 
-  if (!edit->has_prev_log_number_) {
-    edit->SetPrevLogNumber(prev_log_number_);
+  // queue our request
+  ManifestWriter w(mu, edit);
+  manifest_writers_.push_back(&w);
+  while (!w.done && &w != manifest_writers_.front()) {
+    w.cv.Wait();
   }
+  if (w.done) {
+    manifest_lock_.Unlock();
+    return w.status;
+  }
+  
+  std::vector<VersionEdit*> batch_edits;
+  Version* v = new Version(this, current_version_number_++);
+  Builder builder(this, current_);
 
-  edit->SetNextFile(next_file_number_);
-  edit->SetLastSequence(last_sequence_);
-
-  Version* v = new Version(this);
-  {
-    Builder builder(this, current_);
-    builder.Apply(edit);
-    builder.SaveTo(v);
+  // process all requests in the queue
+  ManifestWriter* last_writer = &w;
+  ManifestWriter* first = manifest_writers_.front();
+  assert(!manifest_writers_.empty());
+  assert(first == &w);
+  std::deque<ManifestWriter*>::iterator iter = manifest_writers_.begin();
+  for (; iter != manifest_writers_.end(); ++iter) {
+    last_writer = *iter;
+    LogAndApplyHelper(&builder, v, last_writer->edit, mu);    
+    batch_edits.push_back(last_writer->edit);
   }
-  Finalize(v);
+  builder.SaveTo(v);
 
   // Initialize new descriptor log file if necessary by creating
   // a temporary file that contains a snapshot of the current version.
@@ -800,15 +846,22 @@ Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
     }
   }
 
-  // Unlock during expensive MANIFEST log write
+  // Unlock during expensive MANIFEST log write. New writes cannot get here
+  // because &w is ensuring that all new writes get queued.
   {
     mu->Unlock();
+    Finalize(v);
 
     // Write new record to MANIFEST log
     if (s.ok()) {
       std::string record;
-      edit->EncodeTo(&record);
-      s = descriptor_log_->AddRecord(record);
+      for (unsigned int i = 0; i < batch_edits.size(); i++) {
+        batch_edits[i]->EncodeTo(&record);
+        s = descriptor_log_->AddRecord(record);
+        if (!s.ok()) {
+          break;
+        }
+      }
       if (s.ok()) {
         if (options_->use_fsync) {
           s = descriptor_file_->Fsync();
@@ -826,7 +879,7 @@ Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
 
     // find offset in manifest file where this version is stored.
     new_manifest_file_size = descriptor_file_->GetFileSize();
-
+    
     mu->Lock();
   }
 
@@ -836,7 +889,10 @@ Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
     AppendVersion(v);
     log_number_ = edit->log_number_;
     prev_log_number_ = edit->prev_log_number_;
+
   } else {
+    Log(options_->info_log, "Error in committing version %d", 
+        v->GetVersionNumber());
     delete v;
     if (!new_manifest_file.empty()) {
       delete descriptor_log_;
@@ -847,9 +903,46 @@ Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
     }
   }
 
+  // wake up all the waiting writers
+  while (true) {
+    ManifestWriter* ready = manifest_writers_.front();
+    manifest_writers_.pop_front();
+    if (ready != &w) {
+      ready->status = s;
+      ready->done = true;
+      ready->cv.Signal();
+    }
+    if (ready == last_writer) break;
+  }
+  // Notify new head of write queue
+  if (!manifest_writers_.empty()) {
+    manifest_writers_.front()->cv.Signal();
+  }
   return s;
 }
 
+void VersionSet::LogAndApplyHelper(Builder* builder, Version* v,
+  VersionEdit* edit, port::Mutex* mu) {
+  manifest_lock_.AssertHeld();
+  mu->AssertHeld();
+
+  if (edit->has_log_number_) {
+    assert(edit->log_number_ >= log_number_);
+    assert(edit->log_number_ < next_file_number_);
+  } else {
+    edit->SetLogNumber(log_number_);
+  }
+
+  if (!edit->has_prev_log_number_) {
+    edit->SetPrevLogNumber(prev_log_number_);
+  }
+
+  edit->SetNextFile(next_file_number_);
+  edit->SetLastSequence(last_sequence_);
+
+  builder->Apply(edit);
+}
+
 Status VersionSet::Recover() {
   struct LogReporter : public log::Reader::Reporter {
     Status* status;
@@ -958,7 +1051,7 @@ Status VersionSet::Recover() {
   }
 
   if (s.ok()) {
-    Version* v = new Version(this);
+    Version* v = new Version(this, current_version_number_++);
     builder.SaveTo(v);
     // Install recovered version
     Finalize(v);
@@ -1073,7 +1166,7 @@ Status VersionSet::DumpManifest(Options& options, std::string& dscname) {
   }
 
   if (s.ok()) {
-    Version* v = new Version(this);
+    Version* v = new Version(this, 0);
     builder.SaveTo(v);
     // Install recovered version
     Finalize(v);
@@ -1101,9 +1194,6 @@ void VersionSet::MarkFileNumberUsed(uint64_t number) {
 }
 
 void VersionSet::Finalize(Version* v) {
-  // Precomputed best level for next compaction
-  int best_level = -1;
-  double best_score = -1;
 
   for (int level = 0; level < NumberLevels()-1; level++) {
     double score;
@@ -1119,22 +1209,54 @@ void VersionSet::Finalize(Version* v) {
       // file size is small (perhaps because of a small write-buffer
       // setting, or very high compression ratios, or lots of
       // overwrites/deletions).
-      score = v->files_[level].size() /
+      int numfiles = 0;
+      for (unsigned int i = 0; i < v->files_[level].size(); i++) {
+        if (!v->files_[level][i]->being_compacted) {
+          numfiles++;
+        }
+      }
+
+      // If we are slowing down writes, then we better compact that first
+      if (numfiles >= options_->level0_stop_writes_trigger) {
+        score = 1000000;
+        // Log(options_->info_log, "XXX score l0 = 1000000000 max");
+      } else if (numfiles >= options_->level0_slowdown_writes_trigger) {
+        score = 10000;
+        // Log(options_->info_log, "XXX score l0 = 1000000 medium");
+      } else {
+        score = numfiles /
           static_cast<double>(options_->level0_file_num_compaction_trigger);
+        if (score >= 1) {
+          // Log(options_->info_log, "XXX score l0 = %d least", (int)score);
+        }
+      }
     } else {
       // Compute the ratio of current size to size limit.
-      const uint64_t level_bytes = TotalFileSize(v->files_[level]);
+      const uint64_t level_bytes = TotalFileSize(v->files_[level]) -
+                                   SizeBeingCompacted(level);
       score = static_cast<double>(level_bytes) / MaxBytesForLevel(level);
+      if (score > 1) {
+        // Log(options_->info_log, "XXX score l%d = %d ", level, (int)score);
+      }
     }
-
-    if (score > best_score) {
-      best_level = level;
-      best_score = score;
+    v->compaction_level_[level] = level;
+    v->compaction_score_[level] = score;
+  }
+
+  // sort all the levels based on their score. Higher scores get listed
+  // first. Use bubble sort because the number of entries are small.
+  for(int i = 0; i <  NumberLevels()-2; i++) {
+    for (int j = i+1; j < NumberLevels()-2; j++) {
+      if (v->compaction_score_[i] < v->compaction_score_[j]) {
+        double score = v->compaction_score_[i];
+        int level = v->compaction_level_[i];
+        v->compaction_score_[i] = v->compaction_score_[j];
+        v->compaction_level_[i] = v->compaction_level_[j];
+        v->compaction_score_[j] = score;
+        v->compaction_level_[j] = level;
+      }
     }
   }
-
-  v->compaction_level_ = best_level;
-  v->compaction_score_ = best_score;
 }
 
 Status VersionSet::WriteSnapshot(log::Writer* log) {
@@ -1379,40 +1501,162 @@ int64_t VersionSet::MaxGrandParentOverlapBytes(int level) {
   return result;
 }
 
+// verify that the files listed in this compaction are present 
+// in the current version
+bool VersionSet::VerifyCompactionFileConsistency(Compaction* c) {
+  if (c->input_version_ != current_) {
+    Log(options_->info_log, "VerifyCompactionFileConsistency version mismatch");
+  }
+
+  // verify files in level
+  int level = c->level();
+  for (int i = 0; i < c->num_input_files(0); i++) {
+    uint64_t number = c->input(0,i)->number;
+
+    // look for this file in the current version
+    bool found = false;
+    for (unsigned int j = 0; j < current_->files_[level].size(); j++) {
+      FileMetaData* f = current_->files_[level][j];
+      if (f->number == number) {
+        found = true;
+        break;
+      }
+    }
+    if (!found) {
+      return false; // input files non existant in current version
+    }
+  }
+  // verify level+1 files
+  level++;
+  for (int i = 0; i < c->num_input_files(1); i++) {
+    uint64_t number = c->input(1,i)->number;
+
+    // look for this file in the current version
+    bool found = false;
+    for (unsigned int j = 0; j < current_->files_[level].size(); j++) {
+      FileMetaData* f = current_->files_[level][j];
+      if (f->number == number) {
+        found = true;
+        break;
+      }
+    }
+    if (!found) {
+      return false; // input files non existant in current version
+    }
+  }
+  return true;     // everything good
+}
+
+// Clear all files to indicate that they are not being compacted
+// Delete this compaction from the list of running compactions.
+void VersionSet::ReleaseCompactionFiles(Compaction* c) {
+  c->MarkFilesBeingCompacted(false);
+  compactions_in_progress_[c->level()].erase(c);
+}
+
+// The total size of files that are currently being compacted
+uint64_t VersionSet::SizeBeingCompacted(int level) {
+  uint64_t total = 0;
+  for (std::set<Compaction*>::iterator it = 
+       compactions_in_progress_[level].begin();
+       it != compactions_in_progress_[level].end();
+       ++it) {
+    Compaction* c = (*it); 
+    assert(c->level() == level);
+    for (int i = 0; i < c->num_input_files(0); i++) {
+      total += c->input(0,i)->file_size;
+    }
+  }
+  return total;
+}
+
+Compaction* VersionSet::PickCompactionBySize(int level) {
+  Compaction* c = NULL;
+
+  // level 0 files are overlapping. So we cannot pick more
+  // than one concurrent compactions at this level. This
+  // cold be made better by looking at key-ranges that are
+  // being compacted at level 0.
+  if (level == 0 && compactions_in_progress_[level].size() == 1) {
+    return NULL;
+  }
+
+  assert(level >= 0);
+  assert(level+1 < NumberLevels());
+  c = new Compaction(level, MaxFileSizeForLevel(level),
+      MaxGrandParentOverlapBytes(level), NumberLevels());
+
+  // remember the first file that is not being compacted
+  int firstIndex = 0;
+
+  // Pick the first file that comes after compact_pointer_[level]
+  for (size_t i = 0; i < current_->files_[level].size(); i++) {
+    FileMetaData* f = current_->files_[level][i];
+    // do not pick a file to compact if it is being compacted
+    // from n-1 level.
+    if (f->being_compacted) {
+      continue;
+    }
+    firstIndex = i;
+
+    // Pick a file that has a key-range larger than the range
+    // we picked in the previous call to PickCompaction.
+    if (compact_pointer_[level].empty() ||
+        icmp_.Compare(f->largest.Encode(), compact_pointer_[level]) > 0) {
+      // Do not pick this file if its parents at level+1 are being compacted.
+      // Maybe we can avoid redoing this work in SetupOtherInputs
+      if (ParentFilesInCompaction(f, level)) {
+        continue;
+      }
+      c->inputs_[0].push_back(f);
+      break;
+    }
+  }
+  if (c->inputs_[0].empty()) {
+    // Wrap-around to the beginning of the key space
+    FileMetaData* f = current_->files_[level][firstIndex];
+    if (!f->being_compacted && !ParentFilesInCompaction(f, level)) {
+      c->inputs_[0].push_back(f);
+    }
+  }
+  if (c->inputs_[0].empty()) {
+    delete c;
+    c = NULL;
+  }
+  return c;
+}
+
 Compaction* VersionSet::PickCompaction() {
-  Compaction* c;
+  Compaction* c = NULL;
   int level;
 
+  // compute the compactions needed. It is better to do it here
+  // and also in LogAndApply(), otherwise the values could be stale.
+  Finalize(current_);
+
   // We prefer compactions triggered by too much data in a level over
   // the compactions triggered by seeks.
-  const bool size_compaction = (current_->compaction_score_ >= 1);
-  const bool seek_compaction = (current_->file_to_compact_ != NULL);
-  if (size_compaction) {
-    level = current_->compaction_level_;
-    assert(level >= 0);
-    assert(level+1 < NumberLevels());
-    c = new Compaction(level, MaxFileSizeForLevel(level),
-        MaxGrandParentOverlapBytes(level), NumberLevels());
-
-    // Pick the first file that comes after compact_pointer_[level]
-    for (size_t i = 0; i < current_->files_[level].size(); i++) {
-      FileMetaData* f = current_->files_[level][i];
-      if (compact_pointer_[level].empty() ||
-          icmp_.Compare(f->largest.Encode(), compact_pointer_[level]) > 0) {
-        c->inputs_[0].push_back(f);
+  //
+  // Find the compactions by size on all levels.
+  for (int i = 0; i < NumberLevels()-1; i++) {
+    level = current_->compaction_level_[i];
+    if ((current_->compaction_score_[i] >= 1)) {
+      c = PickCompactionBySize(level);
+      if (c != NULL) {
         break;
       }
     }
-    if (c->inputs_[0].empty()) {
-      // Wrap-around to the beginning of the key space
-      c->inputs_[0].push_back(current_->files_[level][0]);
-    }
-  } else if (seek_compaction) {
+  }
+
+  // Find compactions needed by seeks
+  if (c == NULL && (current_->file_to_compact_ != NULL)) {
     level = current_->file_to_compact_level_;
     c = new Compaction(level, MaxFileSizeForLevel(level),
 		MaxGrandParentOverlapBytes(level), NumberLevels());
     c->inputs_[0].push_back(current_->file_to_compact_);
-  } else {
+  }
+
+  if (c == NULL) {
     return NULL;
   }
 
@@ -1426,15 +1670,46 @@ Compaction* VersionSet::PickCompaction() {
     // Note that the next call will discard the file we placed in
     // c->inputs_[0] earlier and replace it with an overlapping set
     // which will include the picked file.
-    current_->GetOverlappingInputs(0, &smallest, &largest, &c->inputs_[0]);
+    c->inputs_[0].clear();
+    std::vector<FileMetaData*> more;
+    current_->GetOverlappingInputs(0, &smallest, &largest, &more);
+    for (unsigned int i = 0; i < more.size(); i++) {
+      FileMetaData* f = more[i];
+      if (!f->being_compacted && !ParentFilesInCompaction(f, level)) {
+        c->inputs_[0].push_back(f);
+      }
+    }
     assert(!c->inputs_[0].empty());
   }
 
   SetupOtherInputs(c);
 
+  // mark all the files that are being compacted
+  c->MarkFilesBeingCompacted(true);
+
+  // remember this currently undergoing compaction
+  compactions_in_progress_[level].insert(c);
+
   return c;
 }
 
+// Returns true if any one of the parent files are being compacted
+bool VersionSet::ParentFilesInCompaction(FileMetaData* f, int level) {
+  std::vector<FileMetaData*> inputs;
+  current_->GetOverlappingInputs(level+1, &f->smallest, &f->largest, &inputs);
+  return FilesInCompaction(inputs);
+}
+
+// Returns true if any one of specified files are being compacted
+bool VersionSet::FilesInCompaction(std::vector<FileMetaData*>& files) {
+  for (unsigned int i = 0; i < files.size(); i++) {
+    if (files[i]->being_compacted) {
+      return true;
+    } 
+  }
+  return false;
+}
+
 void VersionSet::SetupOtherInputs(Compaction* c) {
   const int level = c->level();
   InternalKey smallest, largest;
@@ -1456,13 +1731,15 @@ void VersionSet::SetupOtherInputs(Compaction* c) {
     const int64_t expanded0_size = TotalFileSize(expanded0);
     int64_t limit = ExpandedCompactionByteSizeLimit(level);
     if (expanded0.size() > c->inputs_[0].size() &&
-        inputs1_size + expanded0_size < limit) {
+        inputs1_size + expanded0_size < limit &&
+        !FilesInCompaction(expanded0)) {
       InternalKey new_start, new_limit;
       GetRange(expanded0, &new_start, &new_limit);
       std::vector<FileMetaData*> expanded1;
       current_->GetOverlappingInputs(level+1, &new_start, &new_limit,
                                      &expanded1);
-      if (expanded1.size() == c->inputs_[1].size()) {
+      if (expanded1.size() == c->inputs_[1].size() && 
+          !FilesInCompaction(expanded1)) {
         Log(options_->info_log,
             "Expanding@%d %d+%d (%ld+%ld bytes) to %d+%d (%ld+%ld bytes)\n",
             level,
@@ -1531,6 +1808,11 @@ Compaction* VersionSet::CompactRange(
   c->input_version_->Ref();
   c->inputs_[0] = inputs;
   SetupOtherInputs(c);
+
+  // These files that are to be manaully compacted do not trample
+  // upon other files because manual compactions are processed when
+  // the system has a max of 1 background compaction thread.
+  c->MarkFilesBeingCompacted(true);
   return c;
 }
 
@@ -1619,6 +1901,18 @@ bool Compaction::ShouldStopBefore(const Slice& internal_key) {
   }
 }
 
+// Mark (or clear) each file that is being compacted
+void Compaction::MarkFilesBeingCompacted(bool value) {
+  for (int i = 0; i < 2; i++) {
+    std::vector<FileMetaData*> v = inputs_[i];
+    for (unsigned int j = 0; j < inputs_[i].size(); j++) {
+      assert(value ? !inputs_[i][j]->being_compacted :
+                      inputs_[i][j]->being_compacted);
+      inputs_[i][j]->being_compacted = value;
+    }
+  }
+}
+
 void Compaction::ReleaseInputs() {
   if (input_version_ != NULL) {
     input_version_->Unref();
@@ -1642,7 +1936,8 @@ static void InputSummary(std::vector<FileMetaData*>& files,
 }
 
 void Compaction::Summary(char* output, int len) {
-  int write = snprintf(output, len, "Base level %d, inputs:", level_);
+  int write = snprintf(output, len, "Base version %ld Base level %d, inputs:", 
+                       input_version_->GetVersionNumber(), level_);
   if(write < 0 || write > len)
     return;
 
diff --git a/db/version_set.h b/db/version_set.h
index c9aa2a5d4..3688cfa5f 100644
--- a/db/version_set.h
+++ b/db/version_set.h
@@ -18,6 +18,7 @@
 #include <map>
 #include <set>
 #include <vector>
+#include <deque>
 #include "db/dbformat.h"
 #include "db/version_edit.h"
 #include "port/port.h"
@@ -106,6 +107,11 @@ class Version {
   // Return a human readable string that describes this version's contents.
   std::string DebugString() const;
 
+  // Returns the version nuber of this version
+  uint64_t GetVersionNumber() {
+    return version_number_;
+  }
+
  private:
   friend class Compaction;
   friend class VersionSet;
@@ -128,13 +134,19 @@ class Version {
   // Level that should be compacted next and its compaction score.
   // Score < 1 means compaction is not strictly needed.  These fields
   // are initialized by Finalize().
-  double compaction_score_;
-  int compaction_level_;
+  // The most critical level to be compacted is listed first
+  // These are used to pick the best compaction level
+  std::vector<double> compaction_score_;
+  std::vector<int> compaction_level_;
 
   // The offset in the manifest file where this version is stored.
   uint64_t offset_manifest_file_;
 
-  explicit Version(VersionSet* vset);
+  // A version number that uniquely represents this version. This is
+  // used for debugging and logging purposes only.
+  uint64_t version_number_;
+
+  explicit Version(VersionSet* vset, uint64_t version_number = 0);
 
   ~Version();
 
@@ -229,10 +241,20 @@ class VersionSet {
   // The caller should delete the iterator when no longer needed.
   Iterator* MakeInputIterator(Compaction* c);
 
+  // Returns true iff some level needs a compaction because it has
+  // exceeded its target size.
+  bool NeedsSizeCompaction() const {
+    for (int i = 0; i < NumberLevels()-1; i++) {
+      if (current_->compaction_score_[i] >= 1) {
+        return true;
+      }
+    }
+    return false;
+  }
   // Returns true iff some level needs a compaction.
   bool NeedsCompaction() const {
-    Version* v = current_;
-    return (v->compaction_score_ >= 1) || (v->file_to_compact_ != NULL);
+    return ((current_->file_to_compact_ != NULL) ||
+            NeedsSizeCompaction());
   }
 
   // Add all files listed in any live version to *live.
@@ -263,8 +285,22 @@ class VersionSet {
   // Return the size of the current manifest file
   const uint64_t ManifestFileSize() { return current_->offset_manifest_file_; }
 
+  // For the specfied level, pick a compaction.
+  // Returns NULL if there is no compaction to be done.
+  Compaction* PickCompactionBySize(int level);
+
+  // Free up the files that were participated in a compaction
+  void ReleaseCompactionFiles(Compaction* c);
+
+  // verify that the files that we started with for a compaction
+  // still exist in the current version and in the same original level.
+  // This ensures that a concurrent compaction did not erroneously
+  // pick the same files to compact.
+  bool VerifyCompactionFileConsistency(Compaction* c);
+
  private:
   class Builder;
+  struct ManifestWriter;
 
   friend class Compaction;
   friend class Version;
@@ -322,9 +358,34 @@ class VersionSet {
   // Per-level max bytes
   uint64_t* level_max_bytes_;
 
+  // record all the ongoing compactions for all levels
+  std::vector<std::set<Compaction*> > compactions_in_progress_;
+
+  // A lock that serialize writes to the manifest
+  port::Mutex manifest_lock_;
+
+  // generates a increasing version number for every new version
+  uint64_t current_version_number_;
+
+  // Queue of writers to the manifest file
+  std::deque<ManifestWriter*> manifest_writers_;
+
   // No copying allowed
   VersionSet(const VersionSet&);
   void operator=(const VersionSet&);
+
+  // Return the total amount of data that is undergoing
+  // compactions at this level
+  uint64_t SizeBeingCompacted(int level);
+
+  // Returns true if any one of the parent files are being compacted
+  bool ParentFilesInCompaction(FileMetaData* f, int level);
+
+  // Returns true if any one of the specified files are being compacted
+  bool FilesInCompaction(std::vector<FileMetaData*>& files);
+
+  void LogAndApplyHelper(Builder*b, Version* v,
+                           VersionEdit* edit, port::Mutex* mu);
 };
 
 // A Compaction encapsulates information about a compaction.
@@ -403,6 +464,9 @@ class Compaction {
   // higher level than the ones involved in this compaction (i.e. for
   // all L >= level_ + 2).
   size_t* level_ptrs_;
+
+  // mark (or clear) all files that are being compacted
+  void MarkFilesBeingCompacted(bool);
 };
 
 }  // namespace leveldb
diff --git a/hdfs/env_hdfs.h b/hdfs/env_hdfs.h
index 944946692..4c07cc63c 100644
--- a/hdfs/env_hdfs.h
+++ b/hdfs/env_hdfs.h
@@ -139,6 +139,10 @@ class HdfsEnv : public Env {
     posixEnv->SetBackgroundThreads(number);
   }
 
+  virtual std::string TimeToString(uint64_t number) {
+    return posixEnv->TimeToString(number);
+  }
+
   static uint64_t gettid() {
     assert(sizeof(pthread_t) <= sizeof(uint64_t));
     return (uint64_t)pthread_self();
@@ -273,6 +277,8 @@ class HdfsEnv : public Env {
       std::string* outputpath) {return notsup;}
 
   virtual void SetBackgroundThreads(int number) {}
+
+  virtual std::string TimeToString(uint64_t number) { return "";}
 };
 }
 
diff --git a/include/leveldb/env.h b/include/leveldb/env.h
index a1a6c854b..f4284ace4 100644
--- a/include/leveldb/env.h
+++ b/include/leveldb/env.h
@@ -159,6 +159,9 @@ class Env {
   // default: 1
   virtual void SetBackgroundThreads(int number) = 0;
 
+  // Converts seconds-since-Jan-01-1970 to a printable string
+  virtual std::string TimeToString(uint64_t time) = 0;
+
  private:
   // No copying allowed
   Env(const Env&);
@@ -358,6 +361,9 @@ class EnvWrapper : public Env {
   void SetBackgroundThreads(int num) {
     return target_->SetBackgroundThreads(num);
   }
+  std::string TimeToString(uint64_t time) {
+    return target_->TimeToString(time);
+  }
 
  private:
   Env* target_;
diff --git a/include/leveldb/options.h b/include/leveldb/options.h
index da26cd440..c3b8aa479 100644
--- a/include/leveldb/options.h
+++ b/include/leveldb/options.h
@@ -78,7 +78,8 @@ struct Options {
   // on disk) before converting to a sorted on-disk file.
   //
   // Larger values increase performance, especially during bulk loads.
-  // Up to two write buffers may be held in memory at the same time,
+  // Up to max_write_buffer_number write buffers may be held in memory 
+  // at the same time,
   // so you may wish to adjust this parameter to control memory usage.
   // Also, a larger write buffer will result in a longer recovery time
   // the next time the database is opened.
@@ -86,6 +87,12 @@ struct Options {
   // Default: 4MB
   size_t write_buffer_size;
 
+  // The maximum number of write buffers that are built up in memory.
+  // The default is 2, so that when 1 write buffer is being flushed to 
+  // storage, new writes can continue to the other write buffer.
+  // Default: 2
+  int max_write_buffer_number;
+
   // Number of open files that can be used by the DB.  You may need to
   // increase this if your database has a large working set (budget
   // one open file per 2MB of working set).
@@ -244,6 +251,10 @@ struct Options {
   // value is 0 which means that obsolete files get removed after
   // every compaction run.
   uint64_t delete_obsolete_files_period_micros;
+  
+  // Maximum number of concurrent background compactions.
+  // Default: 1
+  int max_background_compactions;
 
   // Create an Options object with default values for all fields.
   Options();
diff --git a/tools/db_stress.cc b/tools/db_stress.cc
index 84ab569e4..833ad2c38 100644
--- a/tools/db_stress.cc
+++ b/tools/db_stress.cc
@@ -46,6 +46,16 @@ static bool FLAGS_verbose = false;
 // (initialized to default value by "main")
 static int FLAGS_write_buffer_size = 0;
 
+// The number of in-memory memtables. 
+// Each memtable is of size FLAGS_write_buffer_size.
+// This is initialized to default value of 2 in "main" function.
+static int FLAGS_max_write_buffer_number = 0;
+
+// The maximum number of concurrent background compactions
+// that can occur in parallel.
+// This is initialized to default value of 1 in "main" function.
+static int FLAGS_max_background_compactions = 0;
+
 // Number of bytes to use as a cache of uncompressed data.
 static long FLAGS_cache_size = 2 * KB * KB * KB;
 
@@ -104,6 +114,11 @@ static int FLAGS_readwritepercent = 10;
 // Option to disable compation triggered by read.
 static int FLAGS_disable_seek_compaction = false;
 
+// Option to delete obsolete files periodically
+// Default: 0 which means that obsolete files are
+// deleted after every compaction run.
+ static uint64_t FLAGS_delete_obsolete_files_period_micros = 0;
+
 // Algorithm to use to compress the database
 static enum leveldb::CompressionType FLAGS_compression_type =
     leveldb::kSnappyCompression;
@@ -626,6 +641,8 @@ class StressTest {
     Options options;
     options.block_cache = cache_;
     options.write_buffer_size = FLAGS_write_buffer_size;
+    options.max_write_buffer_number = FLAGS_max_write_buffer_number;
+    options.max_background_compactions = FLAGS_max_background_compactions;
     options.block_size = FLAGS_block_size;
     options.filter_policy = filter_policy_;
     options.max_open_files = FLAGS_open_files;
@@ -644,6 +661,8 @@ class StressTest {
     options.compression = FLAGS_compression_type;
     options.create_if_missing = true;
     options.disable_seek_compaction = FLAGS_disable_seek_compaction;
+    options.delete_obsolete_files_period_micros =
+      FLAGS_delete_obsolete_files_period_micros;
     Status s = DB::Open(options, FLAGS_db, &db_);
     if (!s.ok()) {
       fprintf(stderr, "open error: %s\n", s.ToString().c_str());
@@ -670,7 +689,10 @@ class StressTest {
 
 int main(int argc, char** argv) {
   FLAGS_write_buffer_size = leveldb::Options().write_buffer_size;
+  FLAGS_max_write_buffer_number = leveldb::Options().max_write_buffer_number;
   FLAGS_open_files = leveldb::Options().max_open_files;
+  FLAGS_max_background_compactions =
+    leveldb::Options().max_background_compactions;
   // Compression test code above refers to FLAGS_block_size
   FLAGS_block_size = leveldb::Options().block_size;
   std::string default_db_path;
@@ -706,6 +728,10 @@ int main(int argc, char** argv) {
       FLAGS_value_size_mult = n;
     } else if (sscanf(argv[i], "--write_buffer_size=%d%c", &n, &junk) == 1) {
       FLAGS_write_buffer_size = n;
+    } else if (sscanf(argv[i], "--max_write_buffer_number=%d%c", &n, &junk) == 1) {
+      FLAGS_max_write_buffer_number = n;
+    } else if (sscanf(argv[i], "--max_background_compactions=%d%c", &n, &junk) == 1) {
+      FLAGS_max_background_compactions = n;
     } else if (sscanf(argv[i], "--cache_size=%ld%c", &l, &junk) == 1) {
       FLAGS_cache_size = l;
     } else if (sscanf(argv[i], "--block_size=%d%c", &n, &junk) == 1) {
@@ -784,6 +810,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--disable_seek_compaction=%d%c", &n, &junk) == 1
         && (n == 0 || n == 1)) {
       FLAGS_disable_seek_compaction = n;
+    } else if (sscanf(argv[i], "--delete_obsolete_files_period_micros=%ld%c",
+                      &l, &junk) == 1) {
+      FLAGS_delete_obsolete_files_period_micros = n;
     } else {
       fprintf(stderr, "Invalid flag '%s'\n", argv[i]);
       exit(1);
diff --git a/util/env_posix.cc b/util/env_posix.cc
index 6b5feb284..f2b4a8474 100644
--- a/util/env_posix.cc
+++ b/util/env_posix.cc
@@ -749,6 +749,26 @@ class PosixEnv : public Env {
     }
   }
 
+  virtual std::string TimeToString(uint64_t secondsSince1970) {
+    const time_t seconds = (time_t)secondsSince1970;
+    struct tm t;
+    int maxsize = 64;
+    std::string dummy;
+    dummy.reserve(maxsize);
+    dummy.resize(maxsize);
+    char* p = &dummy[0];
+    localtime_r(&seconds, &t);
+    snprintf(p, maxsize,
+             "%04d/%02d/%02d-%02d:%02d:%02d ",
+             t.tm_year + 1900,
+             t.tm_mon + 1,
+             t.tm_mday,
+             t.tm_hour,
+             t.tm_min,
+             t.tm_sec);
+    return dummy;
+  }
+
  private:
   void PthreadCall(const char* label, int result) {
     if (result != 0) {
diff --git a/util/options.cc b/util/options.cc
index 8078b4db1..5441112b8 100644
--- a/util/options.cc
+++ b/util/options.cc
@@ -19,6 +19,7 @@ Options::Options()
       env(Env::Default()),
       info_log(NULL),
       write_buffer_size(4<<20),
+      max_write_buffer_number(2),
       max_open_files(1000),
       block_cache(NULL),
       block_size(4096),
@@ -42,32 +43,34 @@ Options::Options()
       db_stats_log_interval(1800),
       db_log_dir(""),
       disable_seek_compaction(false),
-      delete_obsolete_files_period_micros(0) {
+      delete_obsolete_files_period_micros(0),
+      max_background_compactions(1) {
 }
 
 void
 Options::Dump(
     Logger * log) const
 {
-    Log(log,"            Options.comparator: %s", comparator->Name());
-    Log(log,"     Options.create_if_missing: %d", create_if_missing);
-    Log(log,"       Options.error_if_exists: %d", error_if_exists);
-    Log(log,"       Options.paranoid_checks: %d", paranoid_checks);
-    Log(log,"                   Options.env: %p", env);
-    Log(log,"              Options.info_log: %p", info_log);
-    Log(log,"     Options.write_buffer_size: %zd", write_buffer_size);
-    Log(log,"        Options.max_open_files: %d", max_open_files);
-    Log(log,"           Options.block_cache: %p", block_cache);
-    Log(log,"      Options.block_cache_size: %zd", block_cache->GetCapacity());
-    Log(log,"            Options.block_size: %zd", block_size);
-    Log(log,"Options.block_restart_interval: %d", block_restart_interval);
-    Log(log,"           Options.compression: %d", compression);
-    Log(log,"         Options.filter_policy: %s",
+    Log(log,"              Options.comparator: %s", comparator->Name());
+    Log(log,"       Options.create_if_missing: %d", create_if_missing);
+    Log(log,"         Options.error_if_exists: %d", error_if_exists);
+    Log(log,"         Options.paranoid_checks: %d", paranoid_checks);
+    Log(log,"                     Options.env: %p", env);
+    Log(log,"                Options.info_log: %p", info_log);
+    Log(log,"       Options.write_buffer_size: %zd", write_buffer_size);
+    Log(log," Options.max_write_buffer_number: %zd", max_write_buffer_number);
+    Log(log,"          Options.max_open_files: %d", max_open_files);
+    Log(log,"             Options.block_cache: %p", block_cache);
+    Log(log,"        Options.block_cache_size: %zd", block_cache->GetCapacity());
+    Log(log,"              Options.block_size: %zd", block_size);
+    Log(log,"  Options.block_restart_interval: %d", block_restart_interval);
+    Log(log,"             Options.compression: %d", compression);
+    Log(log,"           Options.filter_policy: %s",
         filter_policy == NULL ? "NULL" : filter_policy->Name());
-    Log(log,"            Options.num_levels: %d", num_levels);
-    Log(log,"       Options.disableDataSync: %d", disableDataSync);
-    Log(log,"             Options.use_fsync: %d", use_fsync);
-    Log(log," Options.db_stats_log_interval: %d", 
+    Log(log,"              Options.num_levels: %d", num_levels);
+    Log(log,"         Options.disableDataSync: %d", disableDataSync);
+    Log(log,"               Options.use_fsync: %d", use_fsync);
+    Log(log,"   Options.db_stats_log_interval: %d", 
         db_stats_log_interval);
     Log(log,"     Options.level0_file_num_compaction_trigger: %d",
         level0_file_num_compaction_trigger);
@@ -89,10 +92,12 @@ Options::Dump(
         expanded_compaction_factor);
     Log(log,"         Options.max_grandparent_overlap_factor: %d",
         max_grandparent_overlap_factor);
-    Log(log,"         Options.db_log_dir: %s",
+    Log(log,"                             Options.db_log_dir: %s",
         db_log_dir.c_str());
-    Log(log,"         Options.disable_seek_compaction: %d",
+    Log(log,"                Options.disable_seek_compaction: %d",
         disable_seek_compaction);
+    Log(log,"             Options.max_background_compactions: %d",
+        max_background_compactions);
 }   // Options::Dump