Allows Get and MultiGet to read directly from SST files.

Summary:
Add kSstFileTier to ReadTier, which allows Get and MultiGet to
read only directly from SST files and skip mem-tables.

    kSstFileTier = 0x2      // data in SST files.
                          // Note that this ReadTier currently only supports
                          // Get and MultiGet and does not support iterators.

Test Plan: add new test in db_test.

Reviewers: anthony, IslamAbdelRahman, rven, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: igor, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53511

HISTORY.md

@@ -3,12 +3,13 @@
 
 ## 4.5.0 (2/5/2016)
 ### Public API Changes
-* Add a new perf context level between kEnableCount and kEnableTime. Level 2 now doesn't include timers for mutexes.
+* Add a new perf context level between kEnableCount and kEnableTime. Level 2 now does not include timers for mutexes.
 * Statistics of mutex operation durations will not be measured by default. If you want to have them enabled, you need to set Statistics::stats_level_ to kAll.
 * DBOptions::delete_scheduler and NewDeleteScheduler() are removed, please use DBOptions::sst_file_manager and NewSstFileManager() instead
 
 ### New Features
 * ldb tool now supports operations to non-default column families.
+* Add kPersistedTier to ReadTier.  This option allows Get and MultiGet to read only the persited data and skip mem-tables if writes were done with disableWAL = true.
 * Add DBOptions::sst_file_manager. Use NewSstFileManager() in include/rocksdb/sst_file_manager.h to create a SstFileManager that can be used to track the total size of SST files and control the SST files deletion rate.  
 
 ## 4.4.0 (1/14/2016)

db/db_impl.cc

@@ -275,7 +275,7 @@ DBImpl::DBImpl(const DBOptions& options, const std::string& dbname)
           db_options_.delete_obsolete_files_period_micros),
       last_stats_dump_time_microsec_(0),
       next_job_id_(1),
-      flush_on_destroy_(false),
+      has_unpersisted_data_(false),
       env_options_(db_options_),
 #ifndef ROCKSDB_LITE
       wal_manager_(db_options_, env_options_),
@@ -322,7 +322,8 @@ void DBImpl::CancelAllBackgroundWork(bool wait) {
 DBImpl::~DBImpl() {
   mutex_.Lock();
 
-  if (!shutting_down_.load(std::memory_order_acquire) && flush_on_destroy_) {
+  if (!shutting_down_.load(std::memory_order_acquire) &&
+      has_unpersisted_data_) {
     for (auto cfd : *versions_->GetColumnFamilySet()) {
       if (!cfd->IsDropped() && !cfd->mem()->IsEmpty()) {
         cfd->Ref();
@@ -3306,13 +3307,19 @@ Status DBImpl::GetImpl(const ReadOptions& read_options,
   LookupKey lkey(key, snapshot);
   PERF_TIMER_STOP(get_snapshot_time);
 
-  if (sv->mem->Get(lkey, value, &s, &merge_context)) {
-    // Done
-    RecordTick(stats_, MEMTABLE_HIT);
-  } else if (sv->imm->Get(lkey, value, &s, &merge_context)) {
-    // Done
-    RecordTick(stats_, MEMTABLE_HIT);
-  } else {
+  bool skip_memtable =
+      (read_options.read_tier == kPersistedTier && has_unpersisted_data_);
+  bool done = false;
+  if (!skip_memtable) {
+    if (sv->mem->Get(lkey, value, &s, &merge_context)) {
+      done = true;
+      RecordTick(stats_, MEMTABLE_HIT);
+    } else if (sv->imm->Get(lkey, value, &s, &merge_context)) {
+      done = true;
+      RecordTick(stats_, MEMTABLE_HIT);
+    }
+  }
+  if (!done) {
     PERF_TIMER_GUARD(get_from_output_files_time);
     sv->current->Get(read_options, lkey, value, &s, &merge_context,
                      value_found);
@@ -3397,14 +3404,23 @@ std::vector<Status> DBImpl::MultiGet(
     assert(mgd_iter != multiget_cf_data.end());
     auto mgd = mgd_iter->second;
     auto super_version = mgd->super_version;
-    if (super_version->mem->Get(lkey, value, &s, &merge_context)) {
-      // Done
-    } else if (super_version->imm->Get(lkey, value, &s, &merge_context)) {
-      // Done
-    } else {
+    bool skip_memtable =
+        (read_options.read_tier == kPersistedTier && has_unpersisted_data_);
+    bool done = false;
+    if (!skip_memtable) {
+      if (super_version->mem->Get(lkey, value, &s, &merge_context)) {
+        done = true;
+        // TODO(?): RecordTick(stats_, MEMTABLE_HIT)?
+      } else if (super_version->imm->Get(lkey, value, &s, &merge_context)) {
+        done = true;
+        // TODO(?): RecordTick(stats_, MEMTABLE_HIT)?
+      }
+    }
+    if (!done) {
       PERF_TIMER_GUARD(get_from_output_files_time);
       super_version->current->Get(read_options, lkey, value, &s,
                                   &merge_context);
+      // TODO(?): RecordTick(stats_, MEMTABLE_MISS)?
     }
 
     if (s.ok()) {
@@ -3843,6 +3859,10 @@ bool DBImpl::KeyMayExist(const ReadOptions& read_options,
 
 Iterator* DBImpl::NewIterator(const ReadOptions& read_options,
                               ColumnFamilyHandle* column_family) {
+  if (read_options.read_tier == kPersistedTier) {
+    return NewErrorIterator(Status::NotSupported(
+        "ReadTier::kPersistedData is not yet supported in iterators."));
+  }
   auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family);
   auto cfd = cfh->cfd();
 
@@ -3949,6 +3969,10 @@ Status DBImpl::NewIterators(
     const ReadOptions& read_options,
     const std::vector<ColumnFamilyHandle*>& column_families,
     std::vector<Iterator*>* iterators) {
+  if (read_options.read_tier == kPersistedTier) {
+    return Status::NotSupported(
+        "ReadTier::kPersistedData is not yet supported in iterators.");
+  }
   iterators->clear();
   iterators->reserve(column_families.size());
   XFUNC_TEST("", "managed_new", managed_new1, xf_manage_new,
@@ -4327,7 +4351,7 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
     PERF_TIMER_STOP(write_pre_and_post_process_time);
 
     if (write_options.disableWAL) {
-      flush_on_destroy_ = true;
+      has_unpersisted_data_ = true;
     }
 
     uint64_t log_size = 0;

db/db_impl.h

@@ -822,7 +822,10 @@ class DBImpl : public DB {
   // they're unique
   std::atomic<int> next_job_id_;
 
-  bool flush_on_destroy_; // Used when disableWAL is true.
+  // A flag indicating whether the current rocksdb database has any
+  // data that is not yet persisted into either WAL or SST file.
+  // Used when disableWAL is true.
+  bool has_unpersisted_data_;
 
   static const int KEEP_LOG_FILE_NUM = 1000;
   // MSVC version 1800 still does not have constexpr for ::max()

db/db_test.cc

@@ -519,6 +519,135 @@ TEST_F(DBTest, PutSingleDeleteGet) {
                          kSkipUniversalCompaction | kSkipMergePut));
 }
 
+TEST_F(DBTest, ReadFromPersistedTier) {
+  do {
+    Random rnd(301);
+    Options options = CurrentOptions();
+    for (int disableWAL = 0; disableWAL <= 1; ++disableWAL) {
+      CreateAndReopenWithCF({"pikachu"}, options);
+      WriteOptions wopt;
+      wopt.disableWAL = (disableWAL == 1);
+      // 1st round: put but not flush
+      ASSERT_OK(db_->Put(wopt, handles_[1], "foo", "first"));
+      ASSERT_OK(db_->Put(wopt, handles_[1], "bar", "one"));
+      ASSERT_EQ("first", Get(1, "foo"));
+      ASSERT_EQ("one", Get(1, "bar"));
+
+      // Read directly from persited data.
+      ReadOptions ropt;
+      ropt.read_tier = kPersistedTier;
+      std::string value;
+      if (wopt.disableWAL) {
+        // as data has not yet being flushed, we expect not found.
+        ASSERT_TRUE(db_->Get(ropt, handles_[1], "foo", &value).IsNotFound());
+        ASSERT_TRUE(db_->Get(ropt, handles_[1], "bar", &value).IsNotFound());
+      } else {
+        ASSERT_OK(db_->Get(ropt, handles_[1], "foo", &value));
+        ASSERT_OK(db_->Get(ropt, handles_[1], "bar", &value));
+      }
+
+      // Multiget
+      std::vector<ColumnFamilyHandle*> multiget_cfs;
+      multiget_cfs.push_back(handles_[1]);
+      multiget_cfs.push_back(handles_[1]);
+      std::vector<Slice> multiget_keys;
+      multiget_keys.push_back("foo");
+      multiget_keys.push_back("bar");
+      std::vector<std::string> multiget_values;
+      auto statuses =
+          db_->MultiGet(ropt, multiget_cfs, multiget_keys, &multiget_values);
+      if (wopt.disableWAL) {
+        ASSERT_TRUE(statuses[0].IsNotFound());
+        ASSERT_TRUE(statuses[1].IsNotFound());
+      } else {
+        ASSERT_OK(statuses[0]);
+        ASSERT_OK(statuses[1]);
+      }
+
+      // 2nd round: flush and put a new value in memtable.
+      ASSERT_OK(Flush(1));
+      ASSERT_OK(db_->Put(wopt, handles_[1], "rocksdb", "hello"));
+
+      // once the data has been flushed, we are able to get the
+      // data when kPersistedTier is used.
+      ASSERT_TRUE(db_->Get(ropt, handles_[1], "foo", &value).ok());
+      ASSERT_EQ(value, "first");
+      ASSERT_TRUE(db_->Get(ropt, handles_[1], "bar", &value).ok());
+      ASSERT_EQ(value, "one");
+      if (wopt.disableWAL) {
+        ASSERT_TRUE(
+            db_->Get(ropt, handles_[1], "rocksdb", &value).IsNotFound());
+      } else {
+        ASSERT_OK(db_->Get(ropt, handles_[1], "rocksdb", &value));
+        ASSERT_EQ(value, "hello");
+      }
+
+      // Expect same result in multiget
+      multiget_cfs.push_back(handles_[1]);
+      multiget_keys.push_back("rocksdb");
+      statuses =
+          db_->MultiGet(ropt, multiget_cfs, multiget_keys, &multiget_values);
+      ASSERT_TRUE(statuses[0].ok());
+      ASSERT_EQ("first", multiget_values[0]);
+      ASSERT_TRUE(statuses[1].ok());
+      ASSERT_EQ("one", multiget_values[1]);
+      if (wopt.disableWAL) {
+        ASSERT_TRUE(statuses[2].IsNotFound());
+      } else {
+        ASSERT_OK(statuses[2]);
+      }
+
+      // 3rd round: delete and flush
+      ASSERT_OK(db_->Delete(wopt, handles_[1], "foo"));
+      Flush(1);
+      ASSERT_OK(db_->Delete(wopt, handles_[1], "bar"));
+
+      ASSERT_TRUE(db_->Get(ropt, handles_[1], "foo", &value).IsNotFound());
+      if (wopt.disableWAL) {
+        // Still expect finding the value as its delete has not yet being
+        // flushed.
+        ASSERT_TRUE(db_->Get(ropt, handles_[1], "bar", &value).ok());
+        ASSERT_EQ(value, "one");
+      } else {
+        ASSERT_TRUE(db_->Get(ropt, handles_[1], "bar", &value).IsNotFound());
+      }
+      ASSERT_TRUE(db_->Get(ropt, handles_[1], "rocksdb", &value).ok());
+      ASSERT_EQ(value, "hello");
+
+      statuses =
+          db_->MultiGet(ropt, multiget_cfs, multiget_keys, &multiget_values);
+      ASSERT_TRUE(statuses[0].IsNotFound());
+      if (wopt.disableWAL) {
+        ASSERT_TRUE(statuses[1].ok());
+        ASSERT_EQ("one", multiget_values[1]);
+      } else {
+        ASSERT_TRUE(statuses[1].IsNotFound());
+      }
+      ASSERT_TRUE(statuses[2].ok());
+      ASSERT_EQ("hello", multiget_values[2]);
+      if (wopt.disableWAL == 0) {
+        DestroyAndReopen(options);
+      }
+    }
+  } while (ChangeOptions(kSkipHashCuckoo));
+}
+
+TEST_F(DBTest, PersistedTierOnIterator) {
+  // The test needs to be changed if kPersistedTier is supported in iterator.
+  Options options = CurrentOptions();
+  CreateAndReopenWithCF({"pikachu"}, options);
+  ReadOptions ropt;
+  ropt.read_tier = kPersistedTier;
+
+  auto* iter = db_->NewIterator(ropt, handles_[1]);
+  ASSERT_TRUE(iter->status().IsNotSupported());
+  delete iter;
+
+  std::vector<Iterator*> iters;
+  ASSERT_TRUE(db_->NewIterators(ropt, {handles_[1]}, &iters).IsNotSupported());
+  Close();
+}
+
 TEST_F(DBTest, SingleDeleteFlush) {
   // Test to check whether flushing preserves a single delete hidden
   // behind a put.

db/table_cache.cc

@@ -273,7 +273,7 @@ Status TableCache::Get(const ReadOptions& options,
     if (handle != nullptr) {
       ReleaseHandle(handle);
     }
-  } else if (options.read_tier && s.IsIncomplete()) {
+  } else if (options.read_tier == kBlockCacheTier && s.IsIncomplete()) {
     // Couldn't find Table in cache but treat as kFound if no_io set
     get_context->MarkKeyMayExist();
     return Status::OK();

include/rocksdb/options.h

@@ -1294,8 +1294,12 @@ struct Options : public DBOptions, public ColumnFamilyOptions {
 // the block cache. It will not page in data from the OS cache or data that
 // resides in storage.
 enum ReadTier {
-  kReadAllTier = 0x0,    // data in memtable, block cache, OS cache or storage
-  kBlockCacheTier = 0x1  // data in memtable or block cache
+  kReadAllTier = 0x0,     // data in memtable, block cache, OS cache or storage
+  kBlockCacheTier = 0x1,  // data in memtable or block cache
+  kPersistedTier = 0x2    // persisted data.  When WAL is disabled, this option
+                          // will skip data in memtable.
+                          // Note that this ReadTier currently only supports
+                          // Get and MultiGet and does not support iterators.
 };
 
 // Options that control read operations

table/block_based_table_reader.cc

@@ -1255,7 +1255,8 @@ Status BlockBasedTable::Get(const ReadOptions& read_options, const Slice& key,
         BlockIter biter;
         NewDataBlockIterator(rep_, read_options, iiter.value(), &biter);
 
-        if (read_options.read_tier && biter.status().IsIncomplete()) {
+        if (read_options.read_tier == kBlockCacheTier &&
+            biter.status().IsIncomplete()) {
           // couldn't get block from block_cache
           // Update Saver.state to Found because we are only looking for whether
           // we can guarantee the key is not there when "no_io" is set