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

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Date:	Mon, 23 Aug 2010 16:01:15 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"James.Bottomley@...e.de" <James.Bottomley@...e.de>,
	"tytso@....edu" <tytso@....edu>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"swhiteho@...hat.com" <swhiteho@...hat.com>,
	"konishi.ryusuke@....ntt.co.jp" <konishi.ryusuke@....ntt.co.jp>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"vst@...b.net" <vst@...b.net>, "jack@...e.cz" <jack@...e.cz>,
	"hare@...e.de" <hare@...e.de>
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
  sequenced flush

On 2010-08-23 15:58, Ric Wheeler wrote:
> 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.

The problem purely exists on arrays that report write back cache enabled
AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are
they purely urban legend?

-- 
Jens Axboe

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