linux-kernel - Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Message-ID: <46944555.80509@gmail.com>
Date:	Wed, 11 Jul 2007 11:49:57 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Valdis.Kletnieks@...edu
CC:	ric@....com, david@...g.hm, Stefan Bader <Stefan.Bader@...ibm.com>,
	Phillip Susi <psusi@....rr.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	David Chinner <dgc@....com>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices,
 filesystems, and dm/md.

Valdis.Kletnieks@...edu wrote:
> On Tue, 10 Jul 2007 14:39:41 EDT, Ric Wheeler said:
> 
>> All of the high end arrays have non-volatile cache (read, on power loss, it is a 
>> promise that it will get all of your data out to permanent storage). You don't 
>> need to ask this kind of array to drain the cache. In fact, it might just ignore 
>> you if you send it that kind of request ;-)
> 
> OK, I'll bite - how does the kernel know whether the other end of that
> fiberchannel cable is attached to a DMX-3 or to some no-name product that
> may not have the same assurances?  Is there a "I'm a high-end array" bit
> in the sense data that I'm unaware of?

Well, the array just has to tell the kernel that it doesn't to write
back caching.  The kernel automatically selects ORDERED_DRAIN in such case.

-- 
tejun
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