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

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Date:	Fri, 11 Dec 2009 02:14:01 -0500
From:	Oleg Drokin <green@...uxhacker.ru>
To:	linux-ext4@...r.kernel.org
Cc:	Alex Zhuravlev <Alex.Zhuravlev@....COM>,
	Andreas Dilger <adilger@....COM>
Subject: Re: Potential data consistency issue with ASYNC_COMMIT feature

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

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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