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Re: suddenly slow writes on XFS Filesystem

To: stan@xxxxxxxxxxxxxxxxx
Subject: Re: suddenly slow writes on XFS Filesystem
From: Stefan Priebe <s.priebe@xxxxxxxxxxxx>
Date: Mon, 07 May 2012 22:05:27 +0200
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
In-reply-to: <4FA7FA14.6080700@xxxxxxxxxxxxxxxxx>
References: <4FA63DDA.9070707@xxxxxxxxxxxx> <20120507013456.GW5091@dastard> <4FA76E11.1070708@xxxxxxxxxxxx> <20120507071713.GZ5091@dastard> <4FA77842.5010703@xxxxxxxxxxxx> <4FA7FA14.6080700@xxxxxxxxxxxxxxxxx>
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Am 07.05.2012 18:36, schrieb Stan Hoeppner:
This shows what I originally suspected.  Notice how top heavy this
histogram is.  Over half of your free space sits on little islands of
8MB or less.  17% is in islands of 60KB or less.  This is heavily
fragmented free space.  Contrast this with an XFS from the opposite end
of the aging spectrum that is only 1/3rd full and has seen very few
deletes as it has aged:

Notice how it is very bottom heavy, and that 85% of the free space is in
large islands of 16GB to 24GB.
This totally makes sense too me. Thanks for this explanation.

Stefan, at this point in your filesystem's aging process, it may not
matter how much space you keep freeing up, as your deletion of small
files simply adds more heavily fragmented free space to the pool.  It's
the nature of your workload causing this.
This makes sense - do you have any idea or solution for this? Are Filesystems, Block layers or something else which suits this problem / situation?

What I would suggest is doing an xfsdump to a filesystem on another LUN
or machine, expand the size of this LUN by 50% or more (I gather this is
an external RAID), format it appropriately, then xfsrestore.  This will
eliminate your current free space fragmentation, and the 50% size
increase will delay the next occurrence of this problem.  If you can't
expand the LUN, simply do the xfsdump/format/xfsrestore, which will give
you contiguous free space.
But this will only help for a few month or perhaps a year.


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