On Mon, 29 Jan 2007 17:02:14 -0500
"Matthew Kirk" <[email protected]> wrote:
> Regarding the long fsyncs, here's a trace...
>
> I upgraded to a more recent kernel - 2.6.18.6 - and ran it on a workstation.
> This particular box has In this case the elevator is CFQ.
>
> This sample came from a stall that lasted about 2.5 minutes(!) - the longest
> one I've seen yet. The box is a bit more memory constrained than the
> original system but exhibits similar behavior. It doesn't page. Also,
> there is no raid card - simply striped PATA drives.
Using your little test app, the longest fsync() stall I can demonstrate on
2.6.20-rc4-mm1 on plain-old-sata-disk is 1.2 seconds.
What's the max stall you're able to see with the test app?
Perhaps the file is just super-fragmented. If your production app does
something like:
for (a lot) {
fd = open(name);
write(fd, a little bit);
close(fd);
}
in multiple threads, or against a lot of different files then you might be
fragmenting the files a lot. This is because ext3 discards its in-core
anti-fragmentation data structures on close(). So
- Please check your app, see whether or not it is frequently opening and
closing the output files.
- Using `bmap' from
http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz,
determine whether the offending files are highly fragmented.
- Tell us how much dirty data is being written out by these fsyncs?
- Try mounting the filesystem with `-o data=writeback'. Probably won't
help much if it's also demonstrable on ext2.
- Can you reproduce the stalls on a plain-old-disk? Get RAID out of the
picture?
- You've seen the stalls with both CFQ and AS. I guess you could try
deadline and no-op, but it sounds like that won't help.
-
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