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Re: Cleancache support in XFS

To: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Subject: Re: Cleancache support in XFS
From: James Dingwall <james.dingwall@xxxxxxxxxxx>
Date: Fri, 24 May 2013 08:30:16 +0100
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130522192834.GD10617@xxxxxxxxxxxxxxxxxxx>
References: <51810CED.4080003@xxxxxxxxxxx> <20130501162044.GN29359@xxxxxxx> <20130501223022.GQ10481@dastard> <518222D3.3080109@xxxxxxxxxxx> <20130522192834.GD10617@xxxxxxxxxxxxxxxxxxx>
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Konrad Rzeszutek Wilk wrote:
On Thu, May 02, 2013 at 09:24:51AM +0100, James Dingwall wrote:
Dave Chinner wrote:
On Wed, May 01, 2013 at 11:20:44AM -0500, Ben Myers wrote:
Hi James,
Hey folks,
I am walking through my vacation-emails-mbox.

On Wed, May 01, 2013 at 01:39:09PM +0100, James Dingwall wrote:
In reference to: http://oss.sgi.com/archives/xfs/2012-05/msg00046.html

$ grep -r cleancache fs/xfs
on the 3.9 kernel source suggests that no patch was submitted to
enable cleancache for the XFS filesystem.  Since it was suggested
that this could be a one liner I've had a go and my first effort is
inline below.  While this seems to compile OK I have no experience
in filesystems so I would appreciate it if anyone can point out that
it is obviously wrong and likely to eat my data before I try booting
the kernel.

If it seems a reasonable attempt what would be the best way to check
that it isn't doing nasty things?
Hrm.. Looks like there is a doc in Documentation/vm/cleancache.txt which
includes a list of attributes the filesystem needs to have to work properly
with cleancache.
So, those points are:
I had started to look at these too but I feel very out of my depth!
I had similar conclusions to what Dave wrote but I don't think my
thoughts should carry very much (any) weight.  Anyway I gambled and
booted my xen domU with this patch and so far so good...  xen top
shows that tmem is now being used where previously it wasn't.  I'll
try running the xfstests at the weekend after a couple more days up
time to see what happens.
And how did it go?
I am running the patch I created on 3.9.3 on half of my xen guests now and have not noticed any stability or filesystem problems. xl top with 'T' shows that the guests running with it are using ephemeral pages were those without do not. I did do some runs with xfstests which had some failures but they were present with and without the patch. The best I can really offer is that it works for me, ymmv. The patch is available as commit c725011c4fc5d47e12d131f61bd91a58a40036b5 in https://github.com/JKDingwall/linux.git xfs-enable-cleancache or in the first message of this thread.

Regards,
James

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