Re: SSD + RAID

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@vger.kernel.org/msg272253.html
and read two paragraphs further to see how touching
the inode makes ext3 fsync behave differently.