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, 30 May 2007 12:55:14 -0400
From:	Phillip Susi <psusi@....rr.com>
To:	Stefan Bader <Stefan.Bader@...ibm.com>
CC:	Stefan Bader <sbader3@...glemail.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>,
	Tejun Heo <htejun@...il.com>
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices,
 filesystems, and dm/md.

Stefan Bader wrote:
> You got a linear target that consists of two disks. One drive (a)
> supports barriers and the other one (b) doesn't. Device-mapper just
> maps the requests to the appropriate disk. Now the following sequence
> happens:
> 
> 1. block x gets mapped to drive b
> 2. block y (with barrier) gets mapped to drive a
> 
> Since drive a supports barrier request we don't get -EOPNOTSUPP but
> the request with block y might get written before block x since the
> disk are independent. I guess the chances of this are quite low since
> at some point a barrier request will also hit drive b but for the time
> being it might be better to indicate -EOPNOTSUPP right from
> device-mapper.

The device mapper needs to ensure that ALL underlying devices get a 
barrier request when one comes down from above, even if it has to 
construct zero length barriers to send to most of them.

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