slow xfs writes on loopback mounted xfs with dd + seek

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

Using blktrace and seektracer I've captured the I/O on the block device containing the xfs.img and I'm seeing ~55-70 seeks per second during the slow writes, which seems excessive.

I can reproduce this on hardware with 1, 4 or 8 CPUs.

I've testing this with other file systems I and don't see this issue, so it looks like an xfs + loop mounted issue.

Is this a known performance "feature"?

Colin

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Quoted 20 lines · Colin Ian King

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

Quoted 7 lines

..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.

Quoted 40 lines · Brian Foster

Quoted 20 lines · Colin Ian King

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

Quoted 7 lines

..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.

Quoted 6 lines

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.