linux-ext4 - Re: Massive corruption on RAID0

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Message-ID: <87f94c370906300942p7c5f576cgbb8c065625cf1716@mail.gmail.com>
Date:	Tue, 30 Jun 2009 12:42:53 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Krzysztof Kosiński <tweenk.pl@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Massive corruption on RAID0

2009/6/30 Eric Sandeen <sandeen@...hat.com>:
> Krzysztof Kosiński wrote:
<snip>
>>>> By the way, is there some way to have RAID0-like functionality with
>> write barriers?
>
> Mirrors can pass barriers, IIRC, but not stripes (IIRC...) - I don't
> know if any work is being done to address this.
>
I'm pretty sure mdraid is not attempting to address it.

The issue is that barriers with a single drive can simply be sent to
the drive for it to do the heavy lifting.

With raid-0, it is much more difficult.

ie. you send a barrier to 2 different drives.  One drive takes 30
milliseconds to flush the pre-barrier queue to disk and then continues
working on the post barrier data .  The other drive takes 500
milliseconds to do the same.  The end result is out of sync barriers.
Not at all what the filesystem expects.

The only reliable solution is to disable write caching on the drives.
Of course you don't need barriers then.

Greg
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