linux-ext4 - Re: Potential data consistency issue with ASYNC

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Date:	Fri, 11 Dec 2009 10:01:08 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Oleg Drokin <green@...uxhacker.ru>
CC:	linux-ext4@...r.kernel.org,
	Alex Zhuravlev <Alex.Zhuravlev@....COM>,
	Andreas Dilger <adilger@....COM>
Subject: Re: Potential data consistency issue with ASYNC_COMMIT feature

Oleg Drokin wrote:
> Whoops, nevermind, it seems blkdev_issue_flush after commit does the barrier, I see it now.
> It's just rhel5 kernel that is affected.

rhel5 necessarily lags upstream, but updates will come.

We still need to do a lot of actual real-world testing of 
lost caches in -all- ext4 journaling modes, I think.

-Eric

> On Dec 11, 2009, at 1:45 AM, Oleg Drokin wrote:
> 
>> Hello!
>>
>>   I think ext4 ASYNC_COMMIT feature is potentially pretty unsafe 
>>   when write-back cache is enabled on the device.
>>   Since no barriers are ever done with this feature even if
>>   the barriers are enabled, we might end up in the situation
>>   where we write the journal blocks, then commit block, they 
>>   hit the device write-back cache, after that actual metadata
>>   blocks would be allowed to go to disk and eventually they will.
>>
>>   In the end the device might decide to reorder some of the
>>   actual metadata updates in front of journal updates and
>>   if metadata updates will hit the disk and a power or other
>>   failure occurs after that, we have inconsistent filesystem
>>   as a result.
>>
>>   I do not see an easy way to remedy the problem in this case
>>   other than to insert empty barrier after the commit block
>>   and wait for it completion, but I think that would negate
>>   the entire gain from this feature. I wish we actually had
>>   real ordered writes implemented, not just barrier/FUA
>>   sent to the device before every ordered buffer.
>>
>>   Am I missing something?
>>
>>   Thanks.
>>
>> Bye,
>>    Oleg
> 
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