Hello and thanks for RocksDB,
 
When log_write_bench is run with the -bytes_per_sync option, the option does not influence any *sync* behaviour.
 
> strace -e trace=write,sync_file_range ./log_write_bench -record_interval 0 -record_size 1048576 -num_records 11 -bytes_per_sync 2097152 2>&1 | egrep '^(sync|write.*XXXX)'
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
 
I suspect that this is because the bytes_per_sync option now needs to be using a `WritableFileWriter` and not a `WritableFile`.
 
With the diff below applied, it changes to:
 
> strace -e trace=write,sync_file_range ./log_write_bench -record_interval 0 -record_size 1048576 -num_records 11 -bytes_per_sync 2097152 2>&1 | egrep '^(sync|write.*XXXX)'
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
sync_file_range(0x3, 0, 0x200000, 0x2)  = 0
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
sync_file_range(0x3, 0x200000, 0x200000, 0x2) = 0
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
sync_file_range(0x3, 0x400000, 0x200000, 0x2) = 0
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
sync_file_range(0x3, 0x600000, 0x200000, 0x2) = 0
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
write(3, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 1048576) = 1048576
sync_file_range(0x3, 0x800000, 0x200000, 0x2) = 0
 
( Note that the first 1MB is not synced as mentioned in util/file_reader_writer.cc::WritableFileWriter::Flush() )
 
This diff also includes the fix from https://github.com/facebook/rocksdb/pull/1373
 
> diff -du util/log_write_bench.cc.orig util/log_write_bench.cc
--- util/log_write_bench.cc.orig        2016-10-04 12:06:29.115122580 -0400
+++ util/log_write_bench.cc     2016-10-05 07:24:09.677037576 -0400
@@ -14,6 +14,7 @@
 #include <gflags/gflags.h>

 #include "rocksdb/env.h"
+#include "util/file_reader_writer.h"
 #include "util/histogram.h"
 #include "util/testharness.h"
 #include "util/testutil.h"
@@ -38,19 +39,21 @@
   env_options.bytes_per_sync = FLAGS_bytes_per_sync;
   unique_ptr<WritableFile> file;
   env->NewWritableFile(file_name, &file, env_options);
+  unique_ptr<WritableFileWriter> writer;
+  writer.reset(new WritableFileWriter(std::move(file), env_options));

   std::string record;
-  record.assign('X', FLAGS_record_size);
+  record.assign(FLAGS_record_size, 'X');

   HistogramImpl hist;

   uint64_t start_time = env->NowMicros();
   for (int i = 0; i < FLAGS_num_records; i++) {
     uint64_t start_nanos = env->NowNanos();
-    file->Append(record);
-    file->Flush();
+    writer->Append(record);
+    writer->Flush();
     if (FLAGS_enable_sync) {
-      file->Sync();
+      writer->Sync(false);
     }
     hist.Add(env->NowNanos() - start_nanos);