Finer report I/O stats about Flush and Compaction.

Summary:
This diff allows the I/O stats about Flush and Compaction to be reported
in a more accurate way.  Instead of measuring the size of a file, it
measure I/O cost in per read / write basis.

Test Plan: make all check

Reviewers: sdong, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19383

include/rocksdb/iostats_context.h

@@ -0,0 +1,34 @@
+// Copyright (c) 2014, Facebook, Inc.  All rights reserved.
+// This source code is licensed under the BSD-style license found in the
+// LICENSE file in the root directory of this source tree. An additional grant
+// of patent rights can be found in the PATENTS file in the same directory.
+
+#ifndef INCLUDE_ROCKSDB_IOSTATS_CONTEXT_H_
+#define INCLUDE_ROCKSDB_IOSTATS_CONTEXT_H_
+
+#include <stdint.h>
+#include <string>
+
+// A thread local context for gathering io-stats efficiently and transparently.
+namespace rocksdb {
+
+struct IOStatsContext {
+  // reset all io-stats counter to zero
+  void Reset();
+
+  std::string ToString() const;
+
+  // the thread pool id
+  uint64_t thread_pool_id;
+
+  // number of bytes that has been written.
+  uint64_t bytes_written;
+  // number of bytes that has been read.
+  uint64_t bytes_read;
+};
+
+extern __thread IOStatsContext iostats_context;
+
+}  // namespace rocksdb
+
+#endif  // INCLUDE_ROCKSDB_IOSTATS_CONTEXT_H_

include/rocksdb/statistics.h

@@ -119,6 +119,7 @@ enum Tickers {
   WRITE_WITH_WAL,       // Number of Write calls that request WAL
   COMPACT_READ_BYTES,   // Bytes read during compaction
   COMPACT_WRITE_BYTES,  // Bytes written during compaction
+  FLUSH_WRITE_BYTES,    // Bytes written during flush
 
   // Number of table's properties loaded directly from file, without creating
   // table reader object.
@@ -179,6 +180,7 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {WRITE_DONE_BY_OTHER, "rocksdb.write.other"},
     {WRITE_TIMEDOUT, "rocksdb.write.timedout"},
     {WRITE_WITH_WAL, "rocksdb.write.wal"},
+    {FLUSH_WRITE_BYTES, "rocksdb.flush.write.bytes"},
     {COMPACT_READ_BYTES, "rocksdb.compact.read.bytes"},
     {COMPACT_WRITE_BYTES, "rocksdb.compact.write.bytes"},
     {NUMBER_DIRECT_LOAD_TABLE_PROPERTIES,