linux-ext4 - Re: wishful thinking about atomic, multi-sector or full MD stripe width, writes in storage

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Message-ID: <4AA186B0.5090905@redhat.com>
Date:	Fri, 04 Sep 2009 17:29:20 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Mark Lord <lkml@....ca>
CC:	Krzysztof Halasa <khc@...waw.pl>,
	Christoph Hellwig <hch@...radead.org>,
	Michael Tokarev <mjt@....msk.ru>, david@...g.hm,
	Pavel Machek <pavel@....cz>, Theodore Tso <tytso@....edu>,
	NeilBrown <neilb@...e.de>, 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: wishful thinking about atomic, multi-sector or full MD stripe
 width, writes in storage

On 09/04/2009 05:21 PM, Mark Lord wrote:
> Ric Wheeler wrote:
> ..
>> You must disable the write cache on these commodity drives *if* the 
>> MD RAID level does not support barriers properly.
> ..
>
> Rather than further trying to cripple Linux on the notebook,
> (it's bad enough already)..

People using MD on notebooks (not sure there are that many using RAID5 
MD) could leave their write cache enabled.

>
> How about instead, *fixing* the MD layer to properly support barriers?
> That would be far more useful, productive, and better for end-users.
>
> Cheers

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).

In the mean time, if you are using MD to make your data more reliable, I 
would still strongly urge you to disable the write cache when you see 
"barriers disabled" messages spit out in /var/log/messages :-)

ric

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