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

From: Dave Chinner
Date: Thu Jan 27 2011 - 18:42:00 EST

On Thu, Jan 27, 2011 at 10:12:23AM -0500, Mark Lord wrote:
> On 11-01-27 12:30 AM, Stan Hoeppner wrote:
> > Mark Lord put forth on 1/26/2011 9:49 PM:
> >
> >> agcount=7453
> >
> > That's probably a bit high Mark, and very possibly the cause of your problems.
> > :) Unless the disk array backing this filesystem has something like 400-800
> > striped disk drives. You said it's a single 2TB drive right?
> >
> > The default agcount for a single drive filesystem is 4 allocation groups. For
> > mdraid (of any number of disks/configuration) it's 16 allocation groups.
> >
> > Why/how did you end up with 7452 allocation groups? That can definitely cause
> > some performance issues due to massively excessive head seeking, and possibly
> > all manner of weirdness.
> This is great info, exactly the kind of feedback I was hoping for!
> The filesystem is about a year old now, and I probably used agsize=nnnnn
> when creating it or something.
> So if this resulted in what you consider to be many MANY too MANY ags,
> then I can imagine the first new file write wanting to go out and read
> in all of the ag data to determine the "best fit" or something.
> Which might explain some of the delay.
> Once I get the new 2TB drive, I'll re-run mkfs.xfs and then copy everything
> over onto a fresh xfs filesystem.
> Can you recommend a good set of mkfs.xfs parameters to suit the characteristics
> of this system?

And perhaps you want to consider the allocsize mount option, though
that shouldn't be necessary for 2.6.38+...


Dave Chinner
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