direct io write support
...
Summary:
rocksdb direct io support
```
[gzh@dev11575.prn2 ~/rocksdb] ./db_bench -benchmarks=fillseq --num=1000000
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
RocksDB: version 5.0
Date: Wed Nov 23 13:17:43 2016
CPU: 40 * Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz
CPUCache: 25600 KB
Keys: 16 bytes each
Values: 100 bytes each (50 bytes after compression)
Entries: 1000000
Prefix: 0 bytes
Keys per prefix: 0
RawSize: 110.6 MB (estimated)
FileSize: 62.9 MB (estimated)
Write rate: 0 bytes/second
Compression: Snappy
Memtablerep: skip_list
Perf Level: 1
WARNING: Assertions are enabled; benchmarks unnecessarily slow
------------------------------------------------
Initializing RocksDB Options from the specified file
Initializing RocksDB Options from command-line flags
DB path: [/tmp/rocksdbtest-112628/dbbench]
fillseq : 4.393 micros/op 227639 ops/sec; 25.2 MB/s
[gzh@dev11575.prn2 ~/roc
Closes https://github.com/facebook/rocksdb/pull/1564
Differential Revision: D4241093
Pulled By: lightmark
fbshipit-source-id: 98c29e3
Aaron Gao