Add options.manual_wal_flush to db_bench (#9132)

Summary:
It is useful to add options.manual_wal_flush to db_bench

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9132

Test Plan: Run the benchamrk with the option.

Reviewed By: ltamasi

Differential Revision: D32188060

fbshipit-source-id: a70835d3cad0f30095218dfda1daff0a432892e5

tools/db_bench_tool.cc

@@ -759,6 +759,9 @@ DEFINE_bool(use_fsync, false, "If true, issue fsync instead of fdatasync");
 
 DEFINE_bool(disable_wal, false, "If true, do not write WAL for write.");
 
+DEFINE_bool(manual_wal_flush, false,
+            "If true, buffer WAL until buffer is full or a manual FlushWAL().");
+
 DEFINE_string(wal_dir, "", "If not empty, use the given dir for WAL");
 
 DEFINE_string(truth_db, "/dev/shm/truth_db/dbbench",
@@ -3975,6 +3978,7 @@ class Benchmark {
     options.use_direct_reads = FLAGS_use_direct_reads;
     options.use_direct_io_for_flush_and_compaction =
         FLAGS_use_direct_io_for_flush_and_compaction;
+    options.manual_wal_flush = FLAGS_manual_wal_flush;
 #ifndef ROCKSDB_LITE
     options.ttl = FLAGS_fifo_compaction_ttl;
     options.compaction_options_fifo = CompactionOptionsFIFO(