From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: [PATCHES] O_DIRECT for WAL writes |
Date: | 2005-06-24 01:12:16 |
Message-ID: | 200506240112.j5O1CGf12612@candle.pha.pa.us |
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Lists: | pgsql-hackers pgsql-patches |
Tom Lane wrote:
> Greg Stark <gsstark(at)mit(dot)edu> writes:
> > Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> >> Unfortunately, I cannot believe these numbers --- the near equality of
> >> fsync off and fsync on means there is something very wrong with the
> >> measurements. What I suspect is that your ATA drives are doing write
> >> caching and thus the "fsyncs" are not really waiting for I/O at all.
>
> > I wonder whether it would make sense to have an automatic test for this
> > problem. I suspect there are lots of installations out there whose admins
> > don't realize that their hardware is doing this to them.
>
> Not sure about "automatic", but a simple little test program to measure
> the speed of rewriting/fsyncing a small test file would surely be a nice
> thing to have.
>
> The reason I question "automatic" is that you really want to test each
> drive being used, if the system has more than one; but Postgres has no
> idea what the actual hardware layout is, and so no good way to know what
> needs to be tested.
Some folks have battery-backed cached controllers so they would appear
as not handling fsync when in fact they do.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
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