Use special sentinel for default & handle set 0 differently for direc…

… and non-direct IO

db/db_impl/db_impl_open.cc

@@ -143,11 +143,12 @@ DBOptions SanitizeOptions(const std::string& dbname, const DBOptions& src,
  result.wal_dir = result.wal_dir.substr(0, result.wal_dir.size() - 1);
  }
 
- if (result.compaction_readahead_size == 0) {
- if (result.use_direct_reads) {
+ if (result.compaction_readahead_size == Options().compaction_readahead_size ||
+ (result.use_direct_reads && result.compaction_readahead_size == 0)) {
+ if (result.use_direct_reads && result.compaction_readahead_size == 0) {
  TEST_SYNC_POINT_CALLBACK("SanitizeOptions:direct_io", nullptr);
  }
- result.compaction_readahead_size = 1024 * 1024 * 2;
+ result.compaction_readahead_size = 2 * 1024 * 1024;
  }
 
  // Force flush on DB open if 2PC is enabled, since with 2PC we have no

db/db_options_test.cc

@@ -1034,30 +1034,40 @@ TEST_F(DBOptionsTest, SetFIFOCompactionOptions) {
 }
 
 TEST_F(DBOptionsTest, CompactionReadaheadSizeChange) {
- SpecialEnv env(env_);
- Options options;
- options.env = &env;
-
- options.compaction_readahead_size = 0;
- options.level0_file_num_compaction_trigger = 2;
  const std::string kValue(1024, 'v');
- Reopen(options);
-
- ASSERT_EQ(1024 * 1024 * 2,
- dbfull()->GetDBOptions().compaction_readahead_size);
- ASSERT_OK(dbfull()->SetDBOptions({{"compaction_readahead_size", "256"}}));
- ASSERT_EQ(256, dbfull()->GetDBOptions().compaction_readahead_size);
- for (int i = 0; i < 1024; i++) {
- ASSERT_OK(Put(Key(i), kValue));
- }
- ASSERT_OK(Flush());
- for (int i = 0; i < 1024 * 2; i++) {
- ASSERT_OK(Put(Key(i), kValue));
+ for (bool direct_io : {true, false}) {
+ SpecialEnv env(env_);
+ Options options;
+ options.env = &env;
+ options.level0_file_num_compaction_trigger = 2;
+ options.use_direct_reads = direct_io;
+ Reopen(options);
+ // Verify the default value of `Option::compaction_readahead_size`
+ ASSERT_EQ(2 * 1024 * 1024,
+ dbfull()->GetDBOptions().compaction_readahead_size);
+ Close();
+ options.compaction_readahead_size = 0;
+ Reopen(options);
+ // Verify the effect of setting `Option::compaction_readahead_size` to be
+ // zero under direct IO and non-Direct IO
+ ASSERT_EQ(direct_io ? 2 * 1024 * 1024 : 0,
+ dbfull()->GetDBOptions().compaction_readahead_size);
+ // Verify `Option::compaction_readahead_size` is set dynamically to positive
+ // value correctly
+ ASSERT_OK(dbfull()->SetDBOptions({{"compaction_readahead_size", "256"}}));
+ ASSERT_EQ(256, dbfull()->GetDBOptions().compaction_readahead_size);
+ for (int i = 0; i < 1024; i++) {
+ ASSERT_OK(Put(Key(i), kValue));
+ }
+ ASSERT_OK(Flush());
+ for (int i = 0; i < 1024 * 2; i++) {
+ ASSERT_OK(Put(Key(i), kValue));
+ }
+ ASSERT_OK(Flush());
+ ASSERT_OK(dbfull()->TEST_WaitForCompact());
+ ASSERT_EQ(256, env_->compaction_readahead_size_);
+ Close();
  }
- ASSERT_OK(Flush());
- ASSERT_OK(dbfull()->TEST_WaitForCompact());
- ASSERT_EQ(256, env_->compaction_readahead_size_);
- Close();
 }
 
 TEST_F(DBOptionsTest, FIFOTtlBackwardCompatible) {

include/rocksdb/options.h

@@ -952,16 +952,19 @@ struct DBOptions {
  AccessHint access_hint_on_compaction_start = NORMAL;
 
  // The size RocksDB uses to perform readahead during compaction read.
- // If set zero, RocksDB will sanitize it to be 2MB during db open.
  // If you're
  // running RocksDB on spinning disks, you should set this to at least 2MB.
  // That way RocksDB's compaction is doing sequential instead of random reads.
  //
  //
- // Default: 0
+ // Default: 2MB
+ //
+ // For direct IO, if set 0, RocksDB will sanitize the value to the default.
+ // For non-direct IO, if set 0, RocksDB will disable compaction readahead
+ // instead.
  //
  // Dynamically changeable through SetDBOptions() API.
- size_t compaction_readahead_size = 0;
+ size_t compaction_readahead_size = 0xfffffffffffffffe;
 
  // This is a maximum buffer size that is used by WinMmapReadableFile in
  // unbuffered disk I/O mode. We need to maintain an aligned buffer for

unreleased_history/behavior_changes/compaction_readahead_size_option_change.md

@@ -0,0 +1 @@
+`Options::compaction_readahead_size` 's default value is changed from 0 to 2MB. If it's explictly set 0, for direct IO, RocksDB will sanitize the value to the default, while for non-direct IO, RocksDB will disable compaction readahead instead.