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Date:	Sun, 03 Apr 2011 10:37:32 +0800
From:	Tao Ma <tm@....ma>
To:	Ric Wheeler <ricwheeler@...il.com>
CC:	Andreas Dilger <adilger@...ger.ca>,
	Eric Sandeen <sandeen@...hat.com>,
	Daniel Taylor <Daniel.Taylor@....com>,
	"linux-ext4@...r.kernel.org development" <linux-ext4@...r.kernel.org>,
	Johann Lombardi <johann@...mcloud.com>
Subject: Re: breaking ext4 to test recovery

Hi Ric,
On 04/02/2011 08:38 PM, Ric Wheeler wrote:
> On 04/01/2011 10:15 PM, Andreas Dilger wrote:
>> On 2011-03-31, at 12:44 PM, 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?
>> I'm not sure what you mean.  Since the patch works at the block device
>> layer (in __generic_make_request()) it will drop the write at the time
>> it is submitted to the device, not when it is put into the cache.
>>
>> That said, I notice in the linux git repo a line that is in the same
>> place as our patch "if (should_fail_request(bio))" which looks like it
>> might have similar functionality when CONFIG_FAIL_MAKE_REQUEST is
>> enabled.  I'm not sure what kernel version it was added in.  It looks
>> like it is possible to fail the IOs some fraction of the time, or
>> permanently, by writing something into /sys/block/{dev}/fail.
>>
>> Cheers, Andreas
> 
> The device mapper developers are looking at having a device mapper
> target that can be used as a hot block cache - say given a S-ATA disk
> and a PCI-e SSD, you would store the hot blocks on the PCI-e card.
My topic in this year's lsf is "ssd and flashcache". I will talk about
how we use ssd and flashcache in our product system. And one of my
proposal is to rewrite it and get it merged into the upstream. So
you know anyone is working on this by now? I am glad that some other
guys have the same thought as me and we are happy to cooperate with
him/her to get it upstreamed ASAP.

Regards,
Tao
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