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Date: Wed, 02 Sep 2009 20:17:41 +0400
From: Michael Tokarev <mjt@tls.msk.ru>
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To: Pavel Machek <pavel@ucw.cz>
CC: Jeff Garzik <jeff@garzik.org>, Ric Wheeler <rwheeler@redhat.com>,
       Theodore Tso <tytso@mit.edu>, Rob Landley <rob@landley.net>,
       Florian Weimer <fweimer@bfk.de>,
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       kernel list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, mtk.manpages@gmail.com,
       rdunlap@xenotime.net, linux-doc@vger.kernel.org,
       linux-ext4@vger.kernel.org, corbet@lwn.net
Subject: Re: MD/DM and barriers (was Re: [patch] ext2/3: document conditions
 when reliable operation is possible)
References: <20090825225114.GE4300@elf.ucw.cz> <4A948C94.7040103@redhat.com> <20090826025849.GF32712@mit.edu> <200908270019.04231.rob@landley.net> <20090827122423.GO6997@mit.edu> <4A9685D4.2070906@redhat.com> <4A96BA2D.60804@garzik.org> <20090901140120.GD2028@ucw.cz>
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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