linux-kernel - Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Message-ID: <4C727E96.5020801@redhat.com>
Date:	Mon, 23 Aug 2010 09:58:46 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Christoph Hellwig <hch@....de>
CC:	Tejun Heo <tj@...nel.org>, jaxboe@...ionio.com,
	linux-fsdevel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, James.Bottomley@...e.de, tytso@....edu,
	chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, hare@...e.de
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
 sequenced flush

On 08/23/2010 08:48 AM, Christoph Hellwig wrote:
> On Mon, Aug 23, 2010 at 02:30:33PM +0200, Tejun Heo wrote:
>> It might be useful to give several example configurations with
>> different cache configurations.  I don't have much experience with
>> battery backed arrays but aren't they suppose to report write through
>> cache automatically?
>
> They usually do.  I have one that doesn't, but SYNCHRONIZE CACHE on
> it is so fast that it effectively must be a no-op.
>

Arrays are not a problem in general - they normally have internally, redundant 
batteries to hold up the cache.

The issue is when you have an internal hardware RAID card with a large cache. 
Those cards sit in your server and the batteries on the card protect its 
internal cache, but do not have the capacity to hold up the drives behind it.

Normally, those drives should have their write cache disabled, but sometimes 
(especially with S-ATA disks) this is not done.

ric

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