Re: slow xfs writes on loopback mounted xfs with dd + seek

[Dave Chinner <david@fromorbit.com>]

On Tue, Sep 11, 2012 at 10:52:23AM -0400, Brian Foster wrote:
> On 09/11/2012 05:48 AM, Colin Ian King wrote:
> > Hi,
> > 
> > I've seeing really slow I/O writes on xfs when doing a dd with a seek
> > offset to a file on an xfs file system which is loop mounted.
> > 
> > Reproduced on Linux 3.6.0-rc5 and 3.4
> > 
> > How to reproduce:
> > 
> > dd if=/dev/zero of=xfs.img bs=1M count=1024
> > mkfs.xfs -f xfs.img
> > sudo mount -o loop -t xfs xfs.img /mnt/test
> > 
> > First create a large file, write performance is excellent:
> > 
> > sudo dd if=/dev/zero of=/mnt/test/big bs=1M count=500
> > 500+0 records in
> > 500+0 records out
> > 524288000 bytes (524 MB) copied, 1.69451 s, 309 MB/s
> > 
> > ..next seek and write some more blocks, write performance is poor:
> > 
> > sudo dd if=/dev/zero of=/mnt/test/big obs=4K count=8192 seek=131072
> > 8192+0 records in
> > 1024+0 records out
> > 4194304 bytes (4.2 MB) copied, 47.0644 s, 89.1 kB/s
> > 
> 
> Hi Colin,
> 
> I reproduced this behavior with a 1GB filesystem on a loop device. I
> think the problem you're seeing could be circumstantial to the fact that
> you're writing to a point where you are close to filling the fs.
> 
> Taking a look at the tracepoint data when running your second dd alone
> vs. in succession to the first, I see a fairly clean
> buffered_write->get_blocks pattern vs. a
> buffered_write->enospc->log_force->get_blocks pattern.
> 
> In other words, you're triggering an internal space allocation failure
> and flush sequence intended to free up space. Somebody else might be
> able to chime in and more ably explain why that occurs following a
> truncate (perhaps the space isn't freed until the change hits the log),
> but regardless this doesn't seem to occur if you increase the size of
> the fs.

The space is considered "busy" and won't be reused until the
truncate transaction hits the log and the space is free on disk. See
xfs_busy_extent.c

Basically, testing XFS performance on tiny filesystems is going to
show false behaviours. XFS is optimised for large filesystems and
will typically shows low space artifacts on small filesystems,
especially when you are doing things like filling most of the free
filesystem space with 1 file.

e.g.  1GB free on at 100TB filesystem will throttle behaviours (say
speculative preallocation) much more effectively because itis within
1% of ENOSPC. That same 1GB free on a 1GB filesystem won't throttle
preallocation at all, and so that one file when it reaches a little
over 500MB will try to preallocate half the remaining space in the
filesystem because the filesystem is only 50% full....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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