Re: xfs: very slow after mount, very slow at umount

From: Mark Lord
Date: Wed Jan 26 2011 - 22:49:24 EST

On 11-01-26 10:30 PM, Dave Chinner wrote:
> [Please cc xfs@xxxxxxxxxxx on XFS bug reports. Added.]
> On Wed, Jan 26, 2011 at 08:22:25PM -0500, Mark Lord wrote:
>> Alex / Christoph,
>> My mythtv box here uses XFS on a 2TB drive for storing recordings and videos.
>> It is behaving rather strangely though, and has gotten worse recently.
>> Here is what I see happening:
>> The drive mounts fine at boot, but the very first attempt to write a new file
>> to the filesystem suffers from a very very long pause, 30-60 seconds, during which
>> time the disk activity light is fully "on".
> Please post the output of xfs_info <mtpt> so we can see what you
> filesystem configuration is.

/dev/sdb1 on /var/lib/mythtv type xfs

[~] xfs_info /var/lib/mythtv
meta-data=/dev/sdb1 isize=256 agcount=7453, agsize=65536 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=488378638, imaxpct=5
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal bsize=4096 blocks=32768, version=2
= sectsz=512 sunit=0 blks, lazy-count=0
realtime =none extsz=4096 blocks=0, rtextents=0

>> This happens only on the first new file write after mounting.
>> From then on, the filesystem is fast and responsive as expected.
>> If I umount the filesystem, and then mount it again,
>> the exact same behaviour can be observed.
> I can't say I've seen this. Can you capture a blktrace of the IO so
> we can see what IO is actually being done, and perhaps also record
> an XFS event trace as well (i.e. of all the events in
> /sys/kernel/debug/tracing/events/xfs).

I'll have to reconfig/rebuild the kernel to include support for blktrace first.
Can you specify the exact commands/args you'd like for running blktrace etc?

>> This of course screws up mythtv, as it causes me to lose the first 30-60
>> seconds of the first recording it attempts after booting. So as a workaround
>> I now have a startup script to create, sync, and delete a 64MB file before
>> starting mythtv. This still takes 30-60 seconds, but it all happens and
>> finishes before mythtv has a real-time need to write to the filesystem.
>> The 2TB drive is fine -- zero errors, no events in the SMART logs,
>> and I've disabled the silly WD head-unload logic on it.
>> What's happening here? Why the big long burst of activity?
>> I've only just noticed this behaviour in the past few weeks,
>> running 2.6.35 and more recently 2.6.37.
> Can you be a bit more precise? what were you running before 2.6.35
> when you didn't notice this?

Those details are in my earlier follow-up posting.

>> The other issue is something I notice at umount time.

I'm going to let that issue rest for now,
until we figure out the first issue.
Heck, they might even be the exact same thing.. :)

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