Messages in this thread | | | Date | Mon, 29 Jan 2007 15:22:27 -0800 | From | Andrew Morton <> | Subject | Re: fsync occasionally very slow |
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On Mon, 29 Jan 2007 17:02:14 -0500 "Matthew Kirk" <mkirk01@rcn.com> 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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