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Date:	Fri, 01 Apr 2011 11:52:13 -0400
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	Eric Sandeen <sandeen@...hat.com>,
	Andreas Dilger <adilger@...ger.ca>,
	Daniel Taylor <Daniel.Taylor@....com>,
	linux-ext4@...r.kernel.org
Subject: Re: breaking ext4 to test recovery

On 04/01/2011 11:26 AM, Lukas Czerner wrote:
> On Thu, 31 Mar 2011, Eric Sandeen wrote:
>
>> On 3/31/11 5:21 PM, Andreas Dilger wrote:
>>
>>> We have a kernel patch "dev_read_only" that we use with Lustre to
>>> disable writes to the block device while the device is in use.  This
>>> allows simulating crashes at arbitrary points in the code or test
>>> scripts.  It was based on Andrew Morton's test harness that he used
>>> for ext3 recovery testing back when it was being ported to the 2.4
>>> kernel.
>>>
>>> http://git.whamcloud.com/?p=fs/lustre-release.git;a=blob_plain;f=lustre/kernel_patches/patches/dev_read_only-2.6.32-rhel6.patch;hb=HEAD
>>>
>>>   The best part of this patch is that it works with any block device,
>>> can simulate power failure w/o any need for automated power control,
>>> and once the block device is unused (all buffers and references
>>> dropped) it can be re-activated safely.
>> It won't simulate a lost write cache though, will it?
> That's a very good question, I would like to know if there is any way at
> all to force the device to drop the write cache. That would really help
> the power failure testing filesystems.
>
> -Lukas
>

Write cache behavior can be really mysterious. Small writes (say single 4K 
blocks) might stay in cache and not get written for a very long time while 
large, streaming writes might bypass the write cache entirely.

It would be neat to be able to simulate these odd things for failure testing :)

Ric

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