Blob DB: Evict oldest blob file when close to blob db size limit

Summary:
Evict oldest blob file and put it in obsolete_files list when close to blob db size limit. The file will be delete when the `DeleteObsoleteFiles` background job runs next time.
For now I set `kEvictOldestFileAtSize` constant, which controls when to evict the oldest file, at 90%. It could be tweaked or made into an option if really needed; I didn't want to expose it as an option pre-maturely as there are already too many :) .
Closes https://github.com/facebook/rocksdb/pull/3094

Differential Revision: D6187340

Pulled By: sagar0

fbshipit-source-id: 687f8262101b9301bf964b94025a2fe9d8573421

utilities/blob_db/blob_db_impl.cc

@@ -62,7 +62,7 @@ bool blobf_compare_ttl::operator()(const std::shared_ptr<BlobFile>& lhs,
   if (lhs->expiration_range_.first > rhs->expiration_range_.first) {
     return false;
   }
-  return lhs->BlobFileNumber() > rhs->BlobFileNumber();
+  return lhs->BlobFileNumber() < rhs->BlobFileNumber();
 }
 
 void EvictAllVersionsCompactionListener::InternalListener::OnCompaction(
@@ -117,7 +117,8 @@ BlobDBImpl::BlobDBImpl(const std::string& dbname,
       total_periods_ampl_(0),
       total_blob_space_(0),
       open_p1_done_(false),
-      debug_level_(0) {
+      debug_level_(0),
+      oldest_file_evicted_(false) {
   blob_dir_ = (bdb_options_.path_relative)
                   ? dbname + "/" + bdb_options_.blob_dir
                   : bdb_options_.blob_dir;
@@ -171,7 +172,8 @@ BlobDBImpl::BlobDBImpl(DB* db, const BlobDBOptions& blob_db_options)
       last_period_ampl_(0),
       total_periods_write_(0),
       total_periods_ampl_(0),
-      total_blob_space_(0) {
+      total_blob_space_(0),
+      oldest_file_evicted_(false) {
   if (!bdb_options_.blob_dir.empty())
     blob_dir_ = (bdb_options_.path_relative)
                     ? db_->GetName() + "/" + bdb_options_.blob_dir
@@ -931,20 +933,56 @@ uint64_t BlobDBImpl::ExtractExpiration(const Slice& key, const Slice& value,
   return has_expiration ? expiration : kNoExpiration;
 }
 
+std::shared_ptr<BlobFile> BlobDBImpl::GetOldestBlobFile() {
+  std::vector<std::shared_ptr<BlobFile>> blob_files;
+  CopyBlobFiles(&blob_files, [](const std::shared_ptr<BlobFile>& f) {
+    return !f->Obsolete() && f->Immutable();
+  });
+  blobf_compare_ttl compare;
+  return *std::min_element(blob_files.begin(), blob_files.end(), compare);
+}
+
+bool BlobDBImpl::EvictOldestBlobFile() {
+  auto oldest_file = GetOldestBlobFile();
+  if (oldest_file == nullptr) {
+    return false;
+  }
+
+  WriteLock wl(&mutex_);
+  oldest_file->SetCanBeDeleted();
+  obsolete_files_.push_front(oldest_file);
+  oldest_file_evicted_.store(true);
+  return true;
+}
+
+Status BlobDBImpl::CheckSize(size_t blob_size) {
+  uint64_t new_space_util = total_blob_space_.load() + blob_size;
+  if (bdb_options_.blob_dir_size > 0) {
+    if (!bdb_options_.is_fifo &&
+        (new_space_util > bdb_options_.blob_dir_size)) {
+      return Status::NoSpace(
+          "Write failed, as writing it would exceed blob_dir_size limit.");
+    }
+    if (bdb_options_.is_fifo && !oldest_file_evicted_.load() &&
+        (new_space_util >
+         kEvictOldestFileAtSize * bdb_options_.blob_dir_size)) {
+      EvictOldestBlobFile();
+    }
+  }
+
+  return Status::OK();
+}
+
 Status BlobDBImpl::AppendBlob(const std::shared_ptr<BlobFile>& bfile,
                               const std::string& headerbuf, const Slice& key,
                               const Slice& value, uint64_t expiration,
                               std::string* index_entry) {
   auto size_put = BlobLogRecord::kHeaderSize + key.size() + value.size();
-  if (bdb_options_.blob_dir_size > 0 &&
-      (total_blob_space_.load() + size_put) > bdb_options_.blob_dir_size) {
-    if (!bdb_options_.is_fifo) {
-      return Status::NoSpace("Blob DB reached the maximum configured size.");
-    }
+  Status s = CheckSize(size_put);
+  if (!s.ok()) {
+    return s;
   }
 
-  Status s;
-
   uint64_t blob_offset = 0;
   uint64_t key_offset = 0;
   {
@@ -1910,7 +1948,12 @@ std::pair<bool, int64_t> BlobDBImpl::DeleteObsoleteFiles(bool aborted) {
   }
 
   // directory change. Fsync
-  if (file_deleted) dir_ent_->Fsync();
+  if (file_deleted) {
+    dir_ent_->Fsync();
+
+    // reset oldest_file_evicted flag
+    oldest_file_evicted_.store(false);
+  }
 
   // put files back into obsolete if for some reason, delete failed
   if (!tobsolete.empty()) {
@@ -1924,13 +1967,18 @@ std::pair<bool, int64_t> BlobDBImpl::DeleteObsoleteFiles(bool aborted) {
 }
 
 void BlobDBImpl::CopyBlobFiles(
-    std::vector<std::shared_ptr<BlobFile>>* bfiles_copy) {
+    std::vector<std::shared_ptr<BlobFile>>* bfiles_copy,
+    std::function<bool(const std::shared_ptr<BlobFile>&)> predicate) {
   ReadLock rl(&mutex_);
 
-  // take a copy
-  bfiles_copy->reserve(blob_files_.size());
   for (auto const& p : blob_files_) {
-    bfiles_copy->push_back(p.second);
+    bool pred_value = true;
+    if (predicate) {
+      pred_value = predicate(p.second);
+    }
+    if (pred_value) {
+      bfiles_copy->push_back(p.second);
+    }
   }
 }
 

utilities/blob_db/blob_db_impl.h

@@ -205,6 +205,10 @@ class BlobDBImpl : public BlobDB {
   // how often to schedule check seq files period
   static constexpr uint32_t kCheckSeqFilesPeriodMillisecs = 10 * 1000;
 
+  // when should oldest file be evicted:
+  // on reaching 90% of blob_dir_size
+  static constexpr double kEvictOldestFileAtSize = 0.9;
+
   using BlobDB::Put;
   Status Put(const WriteOptions& options, const Slice& key,
              const Slice& value) override;
@@ -414,7 +418,9 @@ class BlobDBImpl : public BlobDB {
   bool FindFileAndEvictABlob(uint64_t file_number, uint64_t key_size,
                              uint64_t blob_offset, uint64_t blob_size);
 
-  void CopyBlobFiles(std::vector<std::shared_ptr<BlobFile>>* bfiles_copy);
+  void CopyBlobFiles(
+      std::vector<std::shared_ptr<BlobFile>>* bfiles_copy,
+      std::function<bool(const std::shared_ptr<BlobFile>&)> predicate = {});
 
   void FilterSubsetOfFiles(
       const std::vector<std::shared_ptr<BlobFile>>& blob_files,
@@ -423,6 +429,12 @@ class BlobDBImpl : public BlobDB {
 
   uint64_t EpochNow() { return env_->NowMicros() / 1000000; }
 
+  Status CheckSize(size_t blob_size);
+
+  std::shared_ptr<BlobFile> GetOldestBlobFile();
+
+  bool EvictOldestBlobFile();
+
   // the base DB
   DBImpl* db_impl_;
   Env* env_;
@@ -526,6 +538,8 @@ class BlobDBImpl : public BlobDB {
   bool open_p1_done_;
 
   uint32_t debug_level_;
+
+  std::atomic<bool> oldest_file_evicted_;
 };
 
 }  // namespace blob_db

utilities/blob_db/blob_db_test.cc

@@ -700,12 +700,15 @@ TEST_F(BlobDBTest, GCExpiredKeyWhileOverwriting) {
   VerifyDB({{"foo", "v2"}});
 }
 
-TEST_F(BlobDBTest, GCOldestSimpleBlobFileWhenOutOfSpace) {
+// This test is no longer valid since we now return an error when we go
+// over the configured blob_dir_size.
+// The test needs to be re-written later in such a way that writes continue
+// after a GC happens.
+TEST_F(BlobDBTest, DISABLED_GCOldestSimpleBlobFileWhenOutOfSpace) {
   // Use mock env to stop wall clock.
   Options options;
   options.env = mock_env_.get();
   BlobDBOptions bdb_options;
-  bdb_options.is_fifo = true;
   bdb_options.blob_dir_size = 100;
   bdb_options.blob_file_size = 100;
   bdb_options.min_blob_size = 0;
@@ -949,6 +952,41 @@ TEST_F(BlobDBTest, OutOfSpace) {
   ASSERT_TRUE(s.IsNoSpace());
 }
 
+TEST_F(BlobDBTest, EvictOldestFileWhenCloseToSpaceLimit) {
+  // Use mock env to stop wall clock.
+  Options options;
+  BlobDBOptions bdb_options;
+  bdb_options.blob_dir_size = 270;
+  bdb_options.blob_file_size = 100;
+  bdb_options.disable_background_tasks = true;
+  bdb_options.is_fifo = true;
+  Open(bdb_options);
+
+  // Each stored blob has an overhead of about 32 bytes currently.
+  // So a 100 byte blob should take up 132 bytes.
+  std::string value(100, 'v');
+  ASSERT_OK(blob_db_->PutWithTTL(WriteOptions(), "key1", value, 10));
+
+  auto *bdb_impl = static_cast<BlobDBImpl *>(blob_db_);
+  auto blob_files = bdb_impl->TEST_GetBlobFiles();
+  ASSERT_EQ(1, blob_files.size());
+
+  // Adding another 100 byte blob would take the total size to 264 bytes
+  // (2*132), which is more than 90% of blob_dir_size. So, the oldest file
+  // should be evicted and put in obsolete files list.
+  ASSERT_OK(blob_db_->PutWithTTL(WriteOptions(), "key2", value, 60));
+
+  auto obsolete_files = bdb_impl->TEST_GetObsoleteFiles();
+  ASSERT_EQ(1, obsolete_files.size());
+  ASSERT_TRUE(obsolete_files[0]->Immutable());
+  ASSERT_EQ(blob_files[0]->BlobFileNumber(),
+            obsolete_files[0]->BlobFileNumber());
+
+  bdb_impl->TEST_DeleteObsoleteFiles();
+  obsolete_files = bdb_impl->TEST_GetObsoleteFiles();
+  ASSERT_TRUE(obsolete_files.empty());
+}
+
 TEST_F(BlobDBTest, InlineSmallValues) {
   constexpr uint64_t kMaxExpiration = 1000;
   Random rnd(301);