RocksDB disable WAL for experimentation. (#8443)

fdbclient/ServerKnobs.cpp

@@ -417,6 +417,9 @@ void ServerKnobs::initialize(Randomize randomize, ClientKnobs* clientKnobs, IsSi
 	init( ROCKSDB_SOFT_PENDING_COMPACT_BYTES_LIMIT,      64000000000 ); // 64GB, Rocksdb option, Writes will slow down.
 	init( ROCKSDB_HARD_PENDING_COMPACT_BYTES_LIMIT,     100000000000 ); // 100GB, Rocksdb option, Writes will stall.
 	init( ROCKSDB_CAN_COMMIT_COMPACT_BYTES_LIMIT,        50000000000 ); // 50GB, Commit waits.
+	// Enable this knob only for experminatal purpose, never enable this in production.
+	// If enabled, all the committed in-memory memtable writes are lost on a crash.
+	init( ROCKSDB_DISABLE_WAL_EXPERIMENTAL,                    false );
 	// Can commit will delay ROCKSDB_CAN_COMMIT_DELAY_ON_OVERLOAD seconds for
 	// ROCKSDB_CAN_COMMIT_DELAY_TIMES_ON_OVERLOAD times, if rocksdb overloaded.
 	// Set ROCKSDB_CAN_COMMIT_DELAY_TIMES_ON_OVERLOAD to 0, to disable

fdbclient/include/fdbclient/ServerKnobs.h

@@ -342,6 +342,7 @@ public:
 	int64_t ROCKSDB_CAN_COMMIT_COMPACT_BYTES_LIMIT;
 	int ROCKSDB_CAN_COMMIT_DELAY_ON_OVERLOAD;
 	int ROCKSDB_CAN_COMMIT_DELAY_TIMES_ON_OVERLOAD;
+	bool ROCKSDB_DISABLE_WAL_EXPERIMENTAL;
 	int64_t ROCKSDB_COMPACTION_READAHEAD_SIZE;
 	int64_t ROCKSDB_BLOCK_SIZE;
 	bool ENABLE_SHARDED_ROCKSDB;

fdbserver/KeyValueStoreRocksDB.actor.cpp

@@ -1195,6 +1195,10 @@ struct RocksDBKeyValueStore : IKeyValueStore {
 			ASSERT(!deletes.empty() || !a.batchToCommit->HasDeleteRange());
 			rocksdb::WriteOptions options;
 			options.sync = !SERVER_KNOBS->ROCKSDB_UNSAFE_AUTO_FSYNC;
+			if (SERVER_KNOBS->ROCKSDB_DISABLE_WAL_EXPERIMENTAL) {
+				options.disableWAL = true;
+				options.sync = false;
+			}
 
 			double writeBeginTime = a.getHistograms ? timer_monotonic() : 0;
 			if (rateLimiter) {