PersistRocksDBOptions() to use WritableFileWriter

Summary:
By using WritableFileWriter rather than WritableFile directly, we can buffer multiple Append() calls to one write() file system call, which will be expensive to underlying Env without its own write buffering.
Closes https://github.com/facebook/rocksdb/pull/3882

Differential Revision: D8080673

Pulled By: siying

fbshipit-source-id: e0db900cb3c178166aa738f3985db65e3ae2cf1b

db/db_options_test.cc

@@ -242,11 +242,12 @@ TEST_F(DBOptionsTest, WritableFileMaxBufferSize) {
   ASSERT_EQ(unmatch_cnt, 0);
   ASSERT_GE(match_cnt, 11);
 
-  buffer_size = 512 * 1024;
-  match_cnt = 0;
-  unmatch_cnt = 0;
   ASSERT_OK(
       dbfull()->SetDBOptions({{"writable_file_max_buffer_size", "524288"}}));
+  buffer_size = 512 * 1024;
+  match_cnt = 0;
+  unmatch_cnt = 0;  // SetDBOptions() will create a WriteableFileWriter
+
   ASSERT_EQ(buffer_size,
             dbfull()->GetDBOptions().writable_file_max_buffer_size);
   i = 0;

options/options_parser.cc

@@ -17,6 +17,7 @@
 #include "rocksdb/convenience.h"
 #include "rocksdb/db.h"
 #include "util/cast_util.h"
+#include "util/file_reader_writer.h"
 #include "util/string_util.h"
 #include "util/sync_point.h"
 
@@ -41,12 +42,16 @@ Status PersistRocksDBOptions(const DBOptions& db_opt,
     return Status::InvalidArgument(
         "cf_names.size() and cf_opts.size() must be the same");
   }
-  std::unique_ptr<WritableFile> writable;
+  std::unique_ptr<WritableFile> wf;
 
-  Status s = env->NewWritableFile(file_name, &writable, EnvOptions());
+  Status s = env->NewWritableFile(file_name, &wf, EnvOptions());
   if (!s.ok()) {
     return s;
   }
+  unique_ptr<WritableFileWriter> writable;
+  writable.reset(new WritableFileWriter(std::move(wf), EnvOptions(),
+                                        nullptr /* statistics */));
+
   std::string options_file_content;
 
   writable->Append(option_file_header + "[" +
@@ -93,8 +98,7 @@ Status PersistRocksDBOptions(const DBOptions& db_opt,
       writable->Append(options_file_content + "\n");
     }
   }
-  writable->Flush();
+  writable->Sync(true /* use_fsync */);
-  writable->Fsync();
   writable->Close();
 
   return RocksDBOptionsParser::VerifyRocksDBOptionsFromFile(