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Date:	Fri, 01 Apr 2011 11:52:13 -0400
From:	Ric Wheeler <>
To:	Lukas Czerner <>
CC:	Eric Sandeen <>,
	Andreas Dilger <>,
	Daniel Taylor <>,
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.
>>>   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 :)


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