linux-kernel - Re: xfs: very slow after mount, very slow at umount

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Date:	Fri, 28 Jan 2011 10:41:52 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Mark Lord <kernel@...savvy.com>
Cc:	Stan Hoeppner <stan@...dwarefreak.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alex Elder <aelder@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: xfs: very slow after mount, very slow at umount

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? 

http://xfs.org/index.php/XFS_FAQ#Q:_I_want_to_tune_my_XFS_filesystems_for_.3Csomething.3E

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

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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