From: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
---|---|
To: | Bruce Momjian <bruce(at)momjian(dot)us> |
Cc: | Greg Smith <greg(at)2ndquadrant(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Brad Nicholson <bnichols(at)ca(dot)afilias(dot)info>, Karl Denninger <karl(at)denninger(dot)net>, Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: SSD + RAID |
Date: | 2009-11-30 00:46:33 |
Message-ID: | 4B1315E9.50400@cheapcomplexdevices.com |
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Lists: | pgsql-performance |
Bruce Momjian wrote:
> Greg Smith wrote:
>> A good test program that is a bit better at introducing and detecting
>> the write cache issue is described at
>> http://brad.livejournal.com/2116715.html
>
> Wow, I had not seen that tool before. I have added a link to it from
> our documentation, and also added a mention of our src/tools/fsync test
> tool to our docs.
One challenge with many of these test programs is that some
filesystem (ext3 is one) will flush drive caches on fsync()
*sometimes, but not always. If your test program happens to do
a sequence of commands that makes an fsync() actually flush a
disk's caches, it might mislead you if your actual application
has a different series of system calls.
For example, ext3 fsync() will issue write barrier commands
if the inode was modified; but not if the inode wasn't.
See test program here:
http://www.mail-archive.com/linux-kernel(at)vger(dot)kernel(dot)org/msg272253.html
and read two paragraphs further to see how touching
the inode makes ext3 fsync behave differently.
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