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Date: Sat, 05 Sep 2009 08:57:57 -0400
From: Mark Lord <lkml@rtr.ca>
Organization: Real-Time Remedies Inc.
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To: Ric Wheeler <rwheeler@redhat.com>
Cc: Krzysztof Halasa <khc@pm.waw.pl>, Christoph Hellwig <hch@infradead.org>,
       Michael Tokarev <mjt@tls.msk.ru>, david@lang.hm,
       Pavel Machek <pavel@ucw.cz>, Theodore Tso <tytso@mit.edu>,
       NeilBrown <neilb@suse.de>, Rob Landley <rob@landley.net>,
       Florian Weimer <fweimer@bfk.de>,
       Goswin von Brederlow <goswin-v-b@web.de>,
       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: wishful thinking about atomic, multi-sector or full MD stripe
 width, writes in storage
References: <20090828064449.GA27528@elf.ucw.cz>	<20090828120854.GA8153@mit.edu> <20090830075135.GA1874@ucw.cz>	<alpine.DEB.2.00.0908300550320.6822@asgard.lang.hm>	<4A9A88B6.9050902@redhat.com> <4A9A9034.8000703@msgid.tls.msk.ru>	<20090830163513.GA25899@infradead.org> <4A9BCCEF.7010402@redhat.com>	<20090831131626.GA17325@infradead.org> <4A9BCDFE.50008@rtr.ca>	<20090831132139.GA5425@infradead.org> <4A9F230F.40707@redhat.com>	<m3ab1cp9ii.fsf@intrepid.localdomain> <4A9FA5F2.9090704@redhat.com>	<m3ljkwnoct.fsf@intrepid.localdomain> <4A9FC9B3.1080809@redhat.com> <m3ab1cnn7y.fsf@intrepid.localdomain> <4A9FCF6B.1080704@redhat.com> <4AA184D7.1010502@rtr.ca> <4AA186B0.5090905@redhat.com>
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Ric Wheeler wrote:
> On 09/04/2009 05:21 PM, Mark Lord wrote:
..
>> How about instead, *fixing* the MD layer to properly support barriers?
>> That would be far more useful, productive, and better for end-users.
..
> Fixing MD would be great - not sure that it would end up still faster 
> (look at md1 devices with working barriers with compared to md1 with 
> write cache disabled).
..

There's no inherent reason for it to be slower, except possibly
drives with b0rked FUA support.

So the first step is to fix MD to pass barriers to the LLDs
for most/all RAID types. 

Then, if it has performance issues, those can be addressed
by more application of little grey cells.  :)

Cheers