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Re: Segmentation fault in xfs_db frag, xfsprogs-3.1.8

To: Richard Ems <richard.ems@xxxxxxxxxxxxxxxxx>
Subject: Re: Segmentation fault in xfs_db frag, xfsprogs-3.1.8
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 6 Jun 2012 23:38:59 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4FCF2BAC.6050100@xxxxxxxxxxxxxxxxx>
References: <4FCC77E1.4030600@xxxxxxxxxxxxxxxxx> <20120605234528.GD22848@dastard> <4FCF2BAC.6050100@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Jun 06, 2012 at 12:06:36PM +0200, Richard Ems wrote:
> On 06/06/2012 01:45 AM, Dave Chinner wrote:
> > No surprise if you have a large filesystem and the filesystem is
> > changing while xfs_db is running. xfs_db is not coherent with
> > mounted filesytems, and it is not recommended that you use it that
> > way.s xfs-db is a debugging tool, not a filesystem state reporting
> > tool.
> 
> Ok, thanks, didn't know that.
> I would like to monitor the fragmentation value for all my mounted XFS.
> I think I read in previous list messages that also other people are
> using xfs_db this way. Or is there another way to get the fragmentation
> value?

No, and what xfs_db reports is mostly useless. You don't have a
fragmentation problem unless you are noticing performance problems,
and no fragmentation number will ever tell you that....

> I found this segmentation fault error very strange, since I have been
> using "echo frag | xfs_db -r" for months already on other big
> filesystems - 2 x 17 TB, 1 x 25 TB, 1 x 20 TB - and NEVER got a
> segmentation fault. All 4 filessystems are mounted read-write and are in
> heavy IO use, nevertheless "echo frag | xfs_db -r" runs as expected (by
> me), no one xfs_db run crashed with a segmentation fault at all for
> months, running every weekend.
> It also ran several times without giving any errors on this 80 TB XFS,
> but then started to throw segmentation faults some weeks ago.

It's entirely possible it is running out of memory, failing to
capture the failure and dereferencing a null pointer....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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