Re: [PERFORM] Direct I/O issues

I have applied your test_fsync patch for 8.2. Thanks.

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Greg Smith wrote:
> I've been trying to optimize a Linux system where benchmarking suggests
> large performance differences between the various wal_sync_method options
> (with o_sync being the big winner). I started that by using
> src/tools/fsync/test_fsync to get an idea what I was dealing with (and to
> spot which drives had write caching turned on). Since those results
> didn't match what I was seeing in the benchmarks, I've been browsing the
> backend source to figure out why. I noticed test_fsync appears to be,
> ahem, out of sync with what the engine is doing.
>
> It looks like V8.1 introduced O_DIRECT writes to the WAL, determined at
> compile time by a series of preprocessor tests in
> src/backend/access/transam/xlog.c When O_DIRECT is available,
> O_SYNC/O_FSYNC/O_DSYNC writes use it. test_fsync doesn't do that.
>
> I moved the new code (in 8.2 beta 3, lines 61-92 in xlog.c) into
> test_fsync; all the flags had the same name so it dropped right in. You
> can get the version I made at http://www.westnet.com/~gsmith/test_fsync.c
> (fixed a compiler warning, too)
>
> The results I get now look fishy. I'm not sure if I screwed up a step, or
> if I'm seeing a real problem. The system here is running RedHat Linux,
> RHEL ES 4.0 kernel 2.6.9, and the disk I'm writing to is a standard
> 7200RPM IDE drive. I turned off write caching with hdparm -W 0
>
> Here's an excerpt from the stock test_fsync:
>
> Compare one o_sync write to two:
> one 16k o_sync write 8.717944
> two 8k o_sync writes 17.501980
>
> Compare file sync methods with 2 8k writes:
> (o_dsync unavailable)
> open o_sync, write 17.018495
> write, fdatasync 8.842473
> write, fsync, 8.809117
>
> And here's the version I tried to modify to include O_DIRECT support:
>
> Compare one o_sync write to two:
> one 16k o_sync write 0.004995
> two 8k o_sync writes 0.003027
>
> Compare file sync methods with 2 8k writes:
> (o_dsync unavailable)
> open o_sync, write 0.004978
> write, fdatasync 8.845498
> write, fsync, 8.834037
>
> Obivously the o_sync writes aren't waiting for the disk. Is this a
> problem with O_DIRECT under Linux? Or is my code just not correctly
> testing this behavior?
>
> Just as a sanity check, I did try this on another system, running SuSE
> with drives connected to a cciss SCSI device, and I got exactly the same
> results. I'm concerned that Linux users who use O_SYNC because they
> notice it's faster will be losing their WAL integrity without being aware
> of the problem, especially as the whole O_DIRECT business isn't even
> mentioned in the WAL documentation--it really deserves to be brought up in
> the wal_sync_method notes at
> http://developer.postgresql.org/pgdocs/postgres/runtime-config-wal.html
>
> And while I'm mentioning improvements to that particular documentation
> page...the wal_buffers notes there are so sparse they misled me initially.
> They suggest only bumping it up for situations with very large
> transactions; since I was testing with small ones I left it woefully
> undersized initially. I would suggest copying the text from
> http://developer.postgresql.org/pgdocs/postgres/wal-configuration.html to
> here: "When full_page_writes is set and the system is very busy, setting
> this value higher will help smooth response times during the period
> immediately following each checkpoint." That seems to match what I found
> in testing.
>
> --
> * Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD
>
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--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +