[RocksDB] Option for incremental sync

Summary: This diff added an option to control the incremenal sync frequency. db_bench has a new flag bytes_per_sync for easy tuning exercise.

Test Plan: make check; db_bench

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11295

db/db_bench.cc

@@ -327,6 +327,12 @@ static bool FLAGS_warn_missing_keys = true;
 static auto FLAGS_use_adaptive_mutex =
   leveldb::Options().use_adaptive_mutex;
 
+// Allows OS to incrementally sync files to disk while they are being
+// written, in the background. Issue one request for every bytes_per_sync
+// written. 0 turns it off.
+static auto FLAGS_bytes_per_sync =
+  leveldb::Options().bytes_per_sync;
+
 namespace leveldb {
 
 // Helper for quickly generating random data.
@@ -1183,6 +1189,7 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
     options.access_hint_on_compaction_start = FLAGS_compaction_fadvice;
 
     options.use_adaptive_mutex = FLAGS_use_adaptive_mutex;
+    options.bytes_per_sync = FLAGS_bytes_per_sync;
 
     Status s;
     if(FLAGS_read_only) {
@@ -2244,6 +2251,8 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--keys_per_multiget=%d%c",
                &n, &junk) == 1) {
       FLAGS_keys_per_multiget = n;
+    }  else if (sscanf(argv[i], "--bytes_per_sync=%ld%c", &l, &junk) == 1) {
+      FLAGS_bytes_per_sync = l;
     } else {
       fprintf(stderr, "Invalid flag '%s'\n", argv[i]);
       exit(1);

include/leveldb/env.h

@@ -55,6 +55,12 @@ struct EnvOptions {
   // If true, set the FD_CLOEXEC on open fd.
   bool set_fd_cloexec;
 
+  // Allows OS to incrementally sync files to disk while they are being
+  // written, in the background. Issue one request for every bytes_per_sync
+  // written. 0 turns it off.
+  // Default: 0
+  uint64_t bytes_per_sync;
+
 };
 
 class Env {

include/leveldb/options.h

@@ -470,6 +470,12 @@ struct Options {
   // Default: false
   bool use_adaptive_mutex;
 
+  // Allows OS to incrementally sync files to disk while they are being
+  // written, asynchronously, in the background.
+  // Issue one request for every bytes_per_sync written. 0 turns it off.
+  // Default: 0
+  uint64_t bytes_per_sync;
+
 };
 
 // Options that control read operations

util/env.cc

@@ -107,6 +107,7 @@ void AssignEnvOptions(EnvOptions* env_options, const Options& options) {
   env_options->use_mmap_reads = options.allow_mmap_reads;
   env_options->use_mmap_writes = options.allow_mmap_writes;
   env_options->set_fd_cloexec = options.is_fd_close_on_exec;
+  env_options->bytes_per_sync = options.bytes_per_sync;
 }
 
 }

util/env_posix.cc

@@ -505,6 +505,7 @@ class PosixWritableFile : public WritableFile {
   bool pending_sync_;
   bool pending_fsync_;
   uint64_t last_sync_size_;
+  uint64_t bytes_per_sync_;
 
  public:
   PosixWritableFile(const std::string& fname, int fd, size_t capacity,
@@ -517,7 +518,8 @@ class PosixWritableFile : public WritableFile {
     filesize_(0),
     pending_sync_(false),
     pending_fsync_(false),
-    last_sync_size_(0) {
+    last_sync_size_(0),
+    bytes_per_sync_(options.bytes_per_sync) {
     assert(!options.use_mmap_writes);
   }
 
@@ -605,8 +607,12 @@ class PosixWritableFile : public WritableFile {
     }
     cursize_ = 0;
 
-    // sync OS cache to disk for every 2MB written
-    if (filesize_ - last_sync_size_ >= 2 * 1024 * 1024) {
+    // sync OS cache to disk for every bytes_per_sync_
+    // TODO: give log file and sst file different options (log
+    // files could be potentially cached in OS for their whole
+    // life time, thus we might not want to flush at all).
+    if (bytes_per_sync_ &&
+        filesize_ - last_sync_size_ >= bytes_per_sync_) {
       RangeSync(last_sync_size_, filesize_ - last_sync_size_);
       last_sync_size_ = filesize_;
     }

util/options.cc

@@ -75,7 +75,8 @@ Options::Options()
       block_size_deviation (10),
       advise_random_on_open(true),
       access_hint_on_compaction_start(NORMAL),
-      use_adaptive_mutex(false) {
+      use_adaptive_mutex(false),
+      bytes_per_sync(0) {
 }
 
 static const char* const access_hints[] = {
@@ -214,6 +215,8 @@ Options::Dump(Logger* log) const
         access_hints[access_hint_on_compaction_start]);
     Log(log,"                      Options.use_adaptive_mutex: %d",
         use_adaptive_mutex);
+    Log(log,"                          Options.bytes_per_sync: %ld",
+        bytes_per_sync);
 }   // Options::Dump
 
 //