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

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Date:	Wed, 11 Jul 2007 18:44:21 -0400
From:	Ric Wheeler <ric@....com>
To:	Valdis.Kletnieks@...edu
CC:	Tejun Heo <htejun@...il.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?
> 

There are ways to query devices (think of hdparm -I in S-ATA/P-ATA drives, SCSI 
has similar queries) to see what kind of device you are talking to. I am not 
sure it is worth the trouble to do any automatic detection/handling of this.

In this specific case, it is more a case of when you attach a high end (or 
mid-tier) device to a server, you should configure it without barriers for its 
exported LUNs.

ric
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