linux-ext4 - Re: MD/DM and barriers (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

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Message-ID: <4A9E9AA5.4020107@msgid.tls.msk.ru>
Date:	Wed, 02 Sep 2009 20:17:41 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Pavel Machek <pavel@....cz>
CC:	Jeff Garzik <jeff@...zik.org>, Ric Wheeler <rwheeler@...hat.com>,
	Theodore Tso <tytso@....edu>, Rob Landley <rob@...dley.net>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: MD/DM and barriers (was Re: [patch] ext2/3: document conditions
 when reliable operation is possible)

Pavel Machek wrote:
> On Thu 2009-08-27 12:54:05, Jeff Garzik wrote:
[]
>> Only md/raid1 supports barriers at present, it seems.  None of the other  
>> MD drivers support barriers.
> 
> Not even md/raid0? Ouch :-(.

Only for raid1 there's no requiriment for inter-drive ordering.  Hence
only raid1 supports barriers (and gained that support very recently,
in 1 or 2 kernel releases).  For the rest, including raid0 and linear,
inter-drive ordering is necessary to implement barriers.  Or md should
have its own queue (flushing) mechanisms.

/mjt
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