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

To: Mark Lord <kernel@xxxxxxxxxxxx>
Subject: Re: xfs: very slow after mount, very slow at umount
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 28 Jan 2011 10:34:09 +1100
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Alex Elder <aelder@xxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <4D40EC2D.5020507@xxxxxxxxxxxx>
References: <4D40C8D1.8090202@xxxxxxxxxxxx> <4D40CDCF.4010301@xxxxxxxxxxxx> <20110127034314.GI21311@dastard> <4D40EC2D.5020507@xxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Jan 26, 2011 at 10:53:17PM -0500, Mark Lord wrote:
> On 11-01-26 10:43 PM, Dave Chinner wrote:
> > On Wed, Jan 26, 2011 at 08:43:43PM -0500, Mark Lord wrote:
> >> On 11-01-26 08:22 PM, Mark Lord wrote:
> ..
> >> Thinking about it some more:  the first problem very much appears as if
> >> it is due to a filesystem check happening on the already-mounted 
> >> filesystem,
> >> if that makes any kind of sense (?).
> > 
> > Not to me.  You can check this simply by looking at the output of
> > top while the problem is occurring...
> 
> Top doesn't show anything interesting, since disk I/O uses practically zero 
> CPU.

My point is that  xfs_check doesn't use zero cpu or memory - it uses
quite a lot of both, so if it is not present in top output while the
disk is being thrashed, it ain't running...

> 
> >> running xfs_check on the umounted drive takes about the same 30-60 seconds,
> >> with the disk activity light fully "on".
> > 
> > Well, yeah - XFS check reads all the metadata in the filesystem, so
> > of course it's going to thrash your disk when it is run. The fact it
> > takes the same length of time as whatever problem you are having is
> > likely to be coincidental.
> 
> I find it interesting that the mount takes zero-time,
> as if it never actually reads much from the filesystem.
> Something has to eventually read the metadata etc.

Sure, for a clean log it has basically nothing to do - a few disk
reads to read the superblock, find the head/tail of the log, and
little else needs doing. Only when log recovery needs to be done
does mount do any significant IO.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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