BlobDB: Remove the need to get sequence number per write

Summary:
Previously we store sequence number range of each blob files, and use the sequence number range to check if the file can be possibly visible by a snapshot. But it adds complexity to the code, since the sequence number is only available after a write. (The current implementation get sequence number by calling GetLatestSequenceNumber(), which is wrong.) With the patch, we are not storing sequence number range, and check if snapshot_sequence < obsolete_sequence to decide if the file is visible by a snapshot (previously we check if first_sequence <= snapshot_sequence < obsolete_sequence).
Closes https://github.com/facebook/rocksdb/pull/3274

Differential Revision: D6571497

Pulled By: yiwu-arbug

fbshipit-source-id: ca06479dc1fcd8782f6525b62b7762cd47d61909

db/db_impl.cc

@@ -1682,12 +1682,6 @@ void DBImpl::ReleaseSnapshot(const Snapshot* s) {
   delete casted_s;
 }
 
-bool DBImpl::HasActiveSnapshotInRange(SequenceNumber lower_bound,
-                                      SequenceNumber upper_bound) {
-  InstrumentedMutexLock l(&mutex_);
-  return snapshots_.HasSnapshotInRange(lower_bound, upper_bound);
-}
-
 #ifndef ROCKSDB_LITE
 Status DBImpl::GetPropertiesOfAllTables(ColumnFamilyHandle* column_family,
                                         TablePropertiesCollection* props) {

db/db_impl.h

@@ -229,10 +229,6 @@ class DBImpl : public DB {
 
   virtual bool SetPreserveDeletesSequenceNumber(SequenceNumber seqnum) override;
 
-  // Whether there is an active snapshot in range [lower_bound, upper_bound).
-  bool HasActiveSnapshotInRange(SequenceNumber lower_bound,
-                                SequenceNumber upper_bound);
-
 #ifndef ROCKSDB_LITE
   using DB::ResetStats;
   virtual Status ResetStats() override;

db/db_write_test.cc

@@ -39,52 +39,6 @@ TEST_P(DBWriteTest, SyncAndDisableWAL) {
   ASSERT_TRUE(dbfull()->Write(write_options, &batch).IsInvalidArgument());
 }
 
-// Sequence number should be return through input write batch.
-TEST_P(DBWriteTest, ReturnSeuqneceNumber) {
-  Random rnd(4422);
-  Open();
-  for (int i = 0; i < 100; i++) {
-    WriteBatch batch;
-    batch.Put("key" + ToString(i), test::RandomHumanReadableString(&rnd, 10));
-    ASSERT_OK(dbfull()->Write(WriteOptions(), &batch));
-    ASSERT_EQ(dbfull()->GetLatestSequenceNumber(),
-              WriteBatchInternal::Sequence(&batch));
-  }
-}
-
-TEST_P(DBWriteTest, ReturnSeuqneceNumberMultiThreaded) {
-  constexpr size_t kThreads = 16;
-  constexpr size_t kNumKeys = 1000;
-  Open();
-  ASSERT_EQ(0, dbfull()->GetLatestSequenceNumber());
-  // Check each sequence is used once and only once.
-  std::vector<std::atomic_flag> flags(kNumKeys * kThreads + 1);
-  for (size_t i = 0; i < flags.size(); i++) {
-    flags[i].clear();
-  }
-  auto writer = [&](size_t id) {
-    Random rnd(4422 + static_cast<uint32_t>(id));
-    for (size_t k = 0; k < kNumKeys; k++) {
-      WriteBatch batch;
-      batch.Put("key" + ToString(id) + "-" + ToString(k),
-                test::RandomHumanReadableString(&rnd, 10));
-      ASSERT_OK(dbfull()->Write(WriteOptions(), &batch));
-      SequenceNumber sequence = WriteBatchInternal::Sequence(&batch);
-      ASSERT_GT(sequence, 0);
-      ASSERT_LE(sequence, kNumKeys * kThreads);
-      // The sequence isn't consumed by someone else.
-      ASSERT_FALSE(flags[sequence].test_and_set());
-    }
-  };
-  std::vector<port::Thread> threads;
-  for (size_t i = 0; i < kThreads; i++) {
-    threads.emplace_back(writer, i);
-  }
-  for (size_t i = 0; i < kThreads; i++) {
-    threads[i].join();
-  }
-}
-
 TEST_P(DBWriteTest, IOErrorOnWALWritePropagateToWriteThreadFollower) {
   constexpr int kNumThreads = 5;
   std::unique_ptr<FaultInjectionTestEnv> mock_env(

db/snapshot_impl.h

@@ -108,22 +108,6 @@ class SnapshotList {
     return ret;
   }
 
-  // Whether there is an active snapshot in range [lower_bound, upper_bound).
-  bool HasSnapshotInRange(SequenceNumber lower_bound,
-                          SequenceNumber upper_bound) {
-    if (empty()) {
-      return false;
-    }
-    const SnapshotImpl* s = &list_;
-    while (s->next_ != &list_) {
-      if (s->next_->number_ >= lower_bound) {
-        return s->next_->number_ < upper_bound;
-      }
-      s = s->next_;
-    }
-    return false;
-  }
-
   // get the sequence number of the most recent snapshot
   SequenceNumber GetNewest() {
     if (empty()) {

utilities/blob_db/blob_db_impl.cc

@@ -571,18 +571,14 @@ class BlobDBImpl::BlobInserter : public WriteBatch::Handler {
   const WriteOptions& options_;
   BlobDBImpl* blob_db_impl_;
   uint32_t default_cf_id_;
-  SequenceNumber sequence_;
   WriteBatch batch_;
 
  public:
   BlobInserter(const WriteOptions& options, BlobDBImpl* blob_db_impl,
-               uint32_t default_cf_id, SequenceNumber seq)
+               uint32_t default_cf_id)
       : options_(options),
         blob_db_impl_(blob_db_impl),
-        default_cf_id_(default_cf_id),
-        sequence_(seq) {}
-
-  SequenceNumber sequence() { return sequence_; }
+        default_cf_id_(default_cf_id) {}
 
   WriteBatch* batch() { return &batch_; }
 
@@ -597,8 +593,7 @@ class BlobDBImpl::BlobInserter : public WriteBatch::Handler {
     uint64_t expiration =
         blob_db_impl_->ExtractExpiration(key, value, &value_slice, &new_value);
     Status s = blob_db_impl_->PutBlobValue(options_, key, value_slice,
-                                           expiration, sequence_, &batch_);
-    sequence_++;
+                                           expiration, &batch_);
     return s;
   }
 
@@ -609,7 +604,6 @@ class BlobDBImpl::BlobInserter : public WriteBatch::Handler {
           "Blob DB doesn't support non-default column family.");
     }
     Status s = WriteBatchInternal::Delete(&batch_, column_family_id, key);
-    sequence_++;
     return s;
   }
 
@@ -621,7 +615,6 @@ class BlobDBImpl::BlobInserter : public WriteBatch::Handler {
     }
     Status s = WriteBatchInternal::DeleteRange(&batch_, column_family_id,
                                                begin_key, end_key);
-    sequence_++;
     return s;
   }
 
@@ -643,12 +636,8 @@ Status BlobDBImpl::Write(const WriteOptions& options, WriteBatch* updates) {
   RecordTick(statistics_, BLOB_DB_NUM_WRITE);
   uint32_t default_cf_id =
       reinterpret_cast<ColumnFamilyHandleImpl*>(DefaultColumnFamily())->GetID();
-  // TODO(yiwu): In case there are multiple writers the latest sequence would
-  // not be the actually sequence we are writting. Need to get the sequence
-  // from write batch after DB write instead.
-  SequenceNumber current_seq = GetLatestSequenceNumber() + 1;
   Status s;
-  BlobInserter blob_inserter(options, this, default_cf_id, current_seq);
+  BlobInserter blob_inserter(options, this, default_cf_id);
   {
     // Release write_mutex_ before DB write to avoid race condition with
     // flush begin listener, which also require write_mutex_ to sync
@@ -693,6 +682,8 @@ Status BlobDBImpl::Write(const WriteOptions& options, WriteBatch* updates) {
 
   if (bdb_options_.enable_garbage_collection) {
     // add deleted key to list of keys that have been deleted for book-keeping
+    SequenceNumber current_seq =
+        WriteBatchInternal::Sequence(blob_inserter.batch());
     DeleteBookkeeper delete_bookkeeper(this, current_seq);
     s = updates->Iterate(&delete_bookkeeper);
   }
@@ -761,11 +752,7 @@ Status BlobDBImpl::PutUntil(const WriteOptions& options, const Slice& key,
     // flush begin listener, which also require write_mutex_ to sync
     // blob files.
     MutexLock l(&write_mutex_);
-    // TODO(yiwu): In case there are multiple writers the latest sequence would
-    // not be the actually sequence we are writting. Need to get the sequence
-    // from write batch after DB write instead.
-    SequenceNumber sequence = GetLatestSequenceNumber() + 1;
-    s = PutBlobValue(options, key, value, expiration, sequence, &batch);
+    s = PutBlobValue(options, key, value, expiration, &batch);
   }
   if (s.ok()) {
     s = db_->Write(options, &batch);
@@ -776,7 +763,7 @@ Status BlobDBImpl::PutUntil(const WriteOptions& options, const Slice& key,
 
 Status BlobDBImpl::PutBlobValue(const WriteOptions& options, const Slice& key,
                                 const Slice& value, uint64_t expiration,
-                                SequenceNumber sequence, WriteBatch* batch) {
+                                WriteBatch* batch) {
   Status s;
   std::string index_entry;
   uint32_t column_family_id =
@@ -817,7 +804,6 @@ Status BlobDBImpl::PutBlobValue(const WriteOptions& options, const Slice& key,
     }
 
     if (s.ok()) {
-      bfile->ExtendSequenceRange(sequence);
       if (expiration != kNoExpiration) {
         bfile->ExtendExpirationRange(expiration);
       }
@@ -898,7 +884,7 @@ bool BlobDBImpl::EvictOldestBlobFile() {
                    total_blob_space_.load(), bdb_options_.blob_dir_size,
                    oldest_file->BlobFileNumber(), expiration_range.first,
                    expiration_range.second);
-    oldest_file->MarkObsolete(oldest_file->GetSequenceRange().second);
+    oldest_file->MarkObsolete(GetLatestSequenceNumber());
     obsolete_files_.push_back(oldest_file);
     oldest_file_evicted_.store(true);
     RecordTick(statistics_, BLOB_DB_FIFO_NUM_FILES_EVICTED);
@@ -1271,9 +1257,26 @@ Status BlobDBImpl::CloseBlobFileIfNeeded(std::shared_ptr<BlobFile>& bfile) {
 bool BlobDBImpl::VisibleToActiveSnapshot(
     const std::shared_ptr<BlobFile>& bfile) {
   assert(bfile->Obsolete());
-  SequenceNumber first_sequence = bfile->GetSequenceRange().first;
+
+  // We check whether the oldest snapshot is no less than the last sequence
+  // by the time the blob file become obsolete. If so, the blob file is not
+  // visible to all existing snapshots.
+  //
+  // If we keep track of the earliest sequence of the keys in the blob file,
+  // we could instead check if there's a snapshot falls in range
+  // [earliest_sequence, obsolete_sequence). But doing so will make the
+  // implementation more complicated.
   SequenceNumber obsolete_sequence = bfile->GetObsoleteSequence();
-  return db_impl_->HasActiveSnapshotInRange(first_sequence, obsolete_sequence);
+  SequenceNumber oldest_snapshot = 0;
+  {
+    // Need to lock DBImpl mutex before access snapshot list.
+    InstrumentedMutexLock l(db_impl_->mutex());
+    auto snapshots = db_impl_->snapshots();
+    if (!snapshots.empty()) {
+      oldest_snapshot = snapshots.oldest()->GetSequenceNumber();
+    }
+  }
+  return oldest_snapshot < obsolete_sequence;
 }
 
 bool BlobDBImpl::FindFileAndEvictABlob(uint64_t file_number, uint64_t key_size,
@@ -1757,8 +1760,6 @@ Status BlobDBImpl::GCFileAndUpdateLSM(const std::shared_ptr<BlobFile>& bfptr,
                                                    &rewrite_batch, &callback);
     }
     if (rewrite_status.ok()) {
-      newfile->ExtendSequenceRange(
-          WriteBatchInternal::Sequence(&rewrite_batch));
       gc_stats->num_keys_relocated++;
       gc_stats->bytes_relocated += record.record_size();
     } else if (rewrite_status.IsBusy()) {
@@ -1775,10 +1776,7 @@ Status BlobDBImpl::GCFileAndUpdateLSM(const std::shared_ptr<BlobFile>& bfptr,
   }  // end of ReadRecord loop
 
   if (s.ok()) {
-    SequenceNumber obsolete_sequence =
-        newfile == nullptr ? bfptr->GetSequenceRange().second + 1
-                           : newfile->GetSequenceRange().second;
-    bfptr->MarkObsolete(obsolete_sequence);
+    bfptr->MarkObsolete(GetLatestSequenceNumber());
     if (!first_gc) {
       WriteLock wl(&mutex_);
       obsolete_files_.push_back(bfptr);

utilities/blob_db/blob_db_impl.h

@@ -284,7 +284,7 @@ class BlobDBImpl : public BlobDB {
 
   Status PutBlobValue(const WriteOptions& options, const Slice& key,
                       const Slice& value, uint64_t expiration,
-                      SequenceNumber sequence, WriteBatch* batch);
+                      WriteBatch* batch);
 
   Status AppendBlob(const std::shared_ptr<BlobFile>& bfile,
                     const std::string& headerbuf, const Slice& key,

utilities/blob_db/blob_db_test.cc

@@ -901,20 +901,18 @@ TEST_F(BlobDBTest, SnapshotAndGarbageCollection) {
       ASSERT_EQ(1, gc_stats.blob_count);
       if (delete_key) {
         ASSERT_EQ(0, gc_stats.num_keys_relocated);
-        ASSERT_EQ(bfile->GetSequenceRange().second + 1,
-                  bfile->GetObsoleteSequence());
       } else {
         ASSERT_EQ(1, gc_stats.num_keys_relocated);
-        ASSERT_EQ(blob_db_->GetLatestSequenceNumber(),
-                  bfile->GetObsoleteSequence());
       }
+      ASSERT_EQ(blob_db_->GetLatestSequenceNumber(),
+                bfile->GetObsoleteSequence());
       if (i == 3) {
         snapshot = blob_db_->GetSnapshot();
       }
       size_t num_files = delete_key ? 3 : 4;
       ASSERT_EQ(num_files, blob_db_impl()->TEST_GetBlobFiles().size());
       blob_db_impl()->TEST_DeleteObsoleteFiles();
-      if (i == 0 || i == 3 || (i == 2 && delete_key)) {
+      if (i == 3) {
         // The snapshot shouldn't see data in bfile
         ASSERT_EQ(num_files - 1, blob_db_impl()->TEST_GetBlobFiles().size());
         blob_db_->ReleaseSnapshot(snapshot);
@@ -1111,10 +1109,6 @@ TEST_F(BlobDBTest, InlineSmallValues) {
   Open(bdb_options, options);
   std::map<std::string, std::string> data;
   std::map<std::string, KeyVersion> versions;
-  SequenceNumber first_non_ttl_seq = kMaxSequenceNumber;
-  SequenceNumber first_ttl_seq = kMaxSequenceNumber;
-  SequenceNumber last_non_ttl_seq = 0;
-  SequenceNumber last_ttl_seq = 0;
   for (size_t i = 0; i < 1000; i++) {
     bool is_small_value = rnd.Next() % 2;
     bool has_ttl = rnd.Next() % 2;
@@ -1134,15 +1128,6 @@ TEST_F(BlobDBTest, InlineSmallValues) {
     versions[key] =
         KeyVersion(key, value, sequence,
                    (is_small_value && !has_ttl) ? kTypeValue : kTypeBlobIndex);
-    if (!is_small_value) {
-      if (!has_ttl) {
-        first_non_ttl_seq = std::min(first_non_ttl_seq, sequence);
-        last_non_ttl_seq = std::max(last_non_ttl_seq, sequence);
-      } else {
-        first_ttl_seq = std::min(first_ttl_seq, sequence);
-        last_ttl_seq = std::max(last_ttl_seq, sequence);
-      }
-    }
   }
   VerifyDB(data);
   VerifyBaseDB(versions);
@@ -1159,11 +1144,7 @@ TEST_F(BlobDBTest, InlineSmallValues) {
     ttl_file = blob_files[1];
   }
   ASSERT_FALSE(non_ttl_file->HasTTL());
-  ASSERT_EQ(first_non_ttl_seq, non_ttl_file->GetSequenceRange().first);
-  ASSERT_EQ(last_non_ttl_seq, non_ttl_file->GetSequenceRange().second);
   ASSERT_TRUE(ttl_file->HasTTL());
-  ASSERT_EQ(first_ttl_seq, ttl_file->GetSequenceRange().first);
-  ASSERT_EQ(last_ttl_seq, ttl_file->GetSequenceRange().second);
 }
 
 TEST_F(BlobDBTest, CompactionFilterNotSupported) {

utilities/blob_db/blob_dump_tool.cc

@@ -142,8 +142,6 @@ Status BlobDumpTool::DumpBlobLogFooter(uint64_t file_size,
   fprintf(stdout, "  Blob count       : %" PRIu64 "\n", footer.blob_count);
   fprintf(stdout, "  Expiration Range : %s\n",
           GetString(footer.expiration_range).c_str());
-  fprintf(stdout, "  Sequence Range   : %s\n",
-          GetString(footer.sequence_range).c_str());
   return s;
 }
 

utilities/blob_db/blob_file.cc

@@ -43,7 +43,6 @@ BlobFile::BlobFile()
       obsolete_(false),
       gc_once_after_open_(false),
       expiration_range_({0, 0}),
-      sequence_range_({kMaxSequenceNumber, 0}),
       last_access_(-1),
       last_fsync_(0),
       header_valid_(false),
@@ -67,7 +66,6 @@ BlobFile::BlobFile(const BlobDBImpl* p, const std::string& bdir, uint64_t fn,
       obsolete_(false),
       gc_once_after_open_(false),
       expiration_range_({0, 0}),
-      sequence_range_({kMaxSequenceNumber, 0}),
       last_access_(-1),
       last_fsync_(0),
       header_valid_(false),
@@ -116,12 +114,11 @@ std::string BlobFile::DumpState() const {
            " file_size: %" PRIu64 " deleted_count: %" PRIu64
            " deleted_size: %" PRIu64
            " closed: %d obsolete: %d expiration_range: (%" PRIu64 ", %" PRIu64
-           ") sequence_range: (%" PRIu64 " %" PRIu64 "), writer: %d reader: %d",
+           "), writer: %d reader: %d",
            path_to_dir_.c_str(), file_number_, blob_count_.load(),
            gc_epoch_.load(), file_size_.load(), deleted_count_, deleted_size_,
            closed_.load(), obsolete_.load(), expiration_range_.first,
-           expiration_range_.second, sequence_range_.first,
-           sequence_range_.second, (!!log_writer_), (!!ra_file_reader_));
+           expiration_range_.second, (!!log_writer_), (!!ra_file_reader_));
   return str;
 }
 
@@ -144,8 +141,6 @@ Status BlobFile::WriteFooterAndCloseLocked() {
     footer.expiration_range = expiration_range_;
   }
 
-  footer.sequence_range = sequence_range_;
-
   // this will close the file and reset the Writable File Pointer.
   Status s = log_writer_->AppendFooter(footer);
   if (s.ok()) {
@@ -185,7 +180,6 @@ Status BlobFile::SetFromFooterLocked(const BlobLogFooter& footer) {
   last_fsync_.store(file_size_);
   blob_count_ = footer.blob_count;
   expiration_range_ = footer.expiration_range;
-  sequence_range_ = footer.sequence_range;
   closed_ = true;
   return Status::OK();
 }

utilities/blob_db/blob_file.h

@@ -84,8 +84,6 @@ class BlobFile {
 
   ExpirationRange expiration_range_;
 
-  SequenceRange sequence_range_;
-
   // Sequential/Append writer for blobs
   std::shared_ptr<Writer> log_writer_;
 
@@ -177,17 +175,6 @@ class BlobFile {
     expiration_range_.second = std::max(expiration_range_.second, expiration);
   }
 
-  SequenceRange GetSequenceRange() const { return sequence_range_; }
-
-  void SetSequenceRange(SequenceRange sequence_range) {
-    sequence_range_ = sequence_range;
-  }
-
-  void ExtendSequenceRange(SequenceNumber sequence) {
-    sequence_range_.first = std::min(sequence_range_.first, sequence);
-    sequence_range_.second = std::max(sequence_range_.second, sequence);
-  }
-
   bool HasTTL() const { return has_ttl_; }
 
   void SetHasTTL(bool has_ttl) { has_ttl_ = has_ttl; }

utilities/blob_db/blob_log_format.cc

@@ -67,8 +67,6 @@ void BlobLogFooter::EncodeTo(std::string* dst) {
   PutFixed64(dst, blob_count);
   PutFixed64(dst, expiration_range.first);
   PutFixed64(dst, expiration_range.second);
-  PutFixed64(dst, sequence_range.first);
-  PutFixed64(dst, sequence_range.second);
   crc = crc32c::Value(dst->c_str(), dst->size());
   crc = crc32c::Mask(crc);
   PutFixed32(dst, crc);
@@ -82,14 +80,12 @@ Status BlobLogFooter::DecodeFrom(Slice src) {
                               "Unexpected blob file footer size");
   }
   uint32_t src_crc = 0;
-  src_crc = crc32c::Value(src.data(), BlobLogFooter::kSize - 4);
+  src_crc = crc32c::Value(src.data(), BlobLogFooter::kSize - sizeof(uint32_t));
   src_crc = crc32c::Mask(src_crc);
   uint32_t magic_number;
   if (!GetFixed32(&src, &magic_number) || !GetFixed64(&src, &blob_count) ||
       !GetFixed64(&src, &expiration_range.first) ||
-      !GetFixed64(&src, &expiration_range.second) ||
-      !GetFixed64(&src, &sequence_range.first) ||
-      !GetFixed64(&src, &sequence_range.second) || !GetFixed32(&src, &crc)) {
+      !GetFixed64(&src, &expiration_range.second) || !GetFixed32(&src, &crc)) {
     return Status::Corruption(kErrorMessage, "Error decoding content");
   }
   if (magic_number != kMagicNumber) {

utilities/blob_db/blob_log_format.h

@@ -24,7 +24,6 @@ constexpr uint32_t kVersion1 = 1;
 constexpr uint64_t kNoExpiration = std::numeric_limits<uint64_t>::max();
 
 using ExpirationRange = std::pair<uint64_t, uint64_t>;
-using SequenceRange = std::pair<uint64_t, uint64_t>;
 
 // Format of blob log file header (30 bytes):
 //
@@ -53,24 +52,23 @@ struct BlobLogHeader {
   Status DecodeFrom(Slice slice);
 };
 
-// Format of blob log file footer (48 bytes):
+// Format of blob log file footer (32 bytes):
 //
-//    +--------------+------------+-------------------+-------------------+------------+
-//    | magic number | blob count | expiration range  |  sequence range   | footer CRC |
-//    +--------------+------------+-------------------+-------------------+------------+
-//    |   Fixed32    |  Fixed64   | Fixed64 + Fixed64 | Fixed64 + Fixed64 |   Fixed32  |
-//    +--------------+------------+-------------------+-------------------+------------+
+//    +--------------+------------+-------------------+------------+
+//    | magic number | blob count | expiration range  | footer CRC |
+//    +--------------+------------+-------------------+------------+
+//    |   Fixed32    |  Fixed64   | Fixed64 + Fixed64 |   Fixed32  |
+//    +--------------+------------+-------------------+------------+
 //
 // The footer will be presented only when the blob file is properly closed.
 //
 // Unlike the same field in file header, expiration range in the footer is the
 // range of smallest and largest expiration of the data in this file.
 struct BlobLogFooter {
-  static constexpr size_t kSize = 48;
+  static constexpr size_t kSize = 32;
 
   uint64_t blob_count = 0;
   ExpirationRange expiration_range = std::make_pair(0, 0);
-  SequenceRange sequence_range = std::make_pair(0, 0);
   uint32_t crc = 0;
 
   void EncodeTo(std::string* dst);