On Tue, Jan 21, 2014 at 09:20:52PM +0100, Jan Kara wrote:
> > If we're forcing the WAL out to disk because of transaction commit or
> > because we need to write the buffer protected by a certain WAL record
> > only after the WAL hits the platter, then it's fine. But sometimes
> > we're writing WAL just because we've run out of internal buffer space,
> > and we don't want to block waiting for the write to complete. Opening
> > the file with O_SYNC deprives us of the ability to control the timing
> > of the sync relative to the timing of the write.
> O_SYNC has a heavy performance penalty. For ext4 it means an extra fs
> transaction commit whenever there's any metadata changed on the filesystem.
> Since mtime & ctime of files will be changed often, the will be a case very
> often.
Also, there is the issue of writes that don't need sycning being synced
because sync is set on the file descriptor. Here is output from our
pg_test_fsync tool when run on an SSD with a BBU:
$ pg_test_fsync5 seconds per testO_DIRECT supported on this platform for open_datasync and open_sync.Compare file sync
methodsusing one 8kB write:(in wal_sync_method preference order, except fdatasyncis Linux's default)
open_datasync n/a fdatasync 8424.785 ops/sec 119
usecs/op fsync 7127.072 ops/sec 140 usecs/op fsync_writethrough
n/a open_sync 10548.469 ops/sec 95 usecs/opCompare file sync
methodsusing two 8kB writes:(in wal_sync_method preference order, except fdatasyncis Linux's default)
open_datasync n/a fdatasync 4367.375 ops/sec 229
usecs/op fsync 4427.761 ops/sec 226 usecs/op fsync_writethrough
n/a open_sync 4303.564 ops/sec 232 usecs/opCompare open_sync with
differentwrite sizes:(This is designed to compare the cost of writing 16kBin different write open_sync sizes.)
--> 1 * 16kB open_sync write 4938.711 ops/sec 202 usecs/op
--> 2 * 8kB open_sync writes 4233.897 ops/sec 236 usecs/op
--> 4 * 4kB open_sync writes 2904.710 ops/sec 344 usecs/op
--> 8 * 2kB open_sync writes 1736.720 ops/sec 576 usecs/op
--> 16 * 1kB open_sync writes 935.917 ops/sec 1068 usecs/opTest if fsync on non-write file
descriptoris honored:(If the times are similar, fsync() can sync data writtenon a different descriptor.) write,
fsync,close 7626.783 ops/sec 131 usecs/op write, close, fsync 6492.697 ops/sec
154 usecs/opNon-Sync'ed 8kB writes: write 351517.178 ops/sec 3 usecs/op
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ Everyone has their own god. +
