From: Krzysztof Halasa <khc@pm.waw.pl>
Subject: Re: wishful thinking about atomic, multi-sector or full MD stripe width, writes in storage
Date: Fri, 04 Sep 2009 01:50:42 +0200
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Cc: Christoph Hellwig <hch@infradead.org>, Mark Lord <lkml@rtr.ca>,
	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
To: Ric Wheeler <rwheeler@redhat.com>
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In-Reply-To: <4A9FCF6B.1080704@redhat.com> (Ric Wheeler's message of "Thu, 03
	Sep 2009 10:15:07 -0400")
Sender: linux-doc-owner@vger.kernel.org
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Ric Wheeler <rwheeler@redhat.com> writes:

> The whole thread above is about software MD using commodity drives
> (S-ATA or SAS) without battery backed write cache.

Yes. However, you mentioned external RAID arrays disable disk caches.
That's why I asked if they are using SATA or SCSI/etc. disks, and if
they have battery-backed cache.

> Also, when you enable the write cache (MD or not) you are buffering
> multiple MB's of data that can go away on power loss. Far greater
> (10x) the exposure that the partial RAID rewrite case worries about.

The cache is flushed with working barriers. I guess it should be
superior to disabled WB cache, in both performance and expected disk
lifetime.
-- 
Krzysztof Halasa