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Date:	Wed, 28 Oct 2009 20:19:01 +0000
From:	Andrew Patterson <>
To:	Mikulas Patocka <>
Cc:	Alasdair G Kergon <>,
	Mark McLoughlin <>,
	device-mapper development <>,
	Jens Axboe <>,
	Rusty Russell <>,
	linux-kernel <>,
	virtualization <>,
	". troy.heber"
Subject: Re: [dm-devel] [PATCH] block: silently error unsupported empty
 barriers too

On Thu, 2009-08-06 at 21:50 -0400, Mikulas Patocka wrote:
> On Thu, 6 Aug 2009, Alasdair G Kergon wrote:
> > On Thu, Aug 06, 2009 at 12:14:17PM +0100, Mark McLoughlin wrote:
> > > We should error all barriers, even empty barriers, on devices like
> > > virtio_blk which don't support them.
> >  
> > Have you considered whether or not virtio_blk actually needs to
> > support empty barriers?
> > 
> > Alasdair
> This is only for request-based drivers, where it is the responsibility of 
> blk-core to translate barriers. I think the empty barrier request anyway 
> in blk_do_ordered, but with an error message. So the patch changes it to 
> discard it early and queitly. It seems ok.
> Mikulas
> --

This patch seems to have been dropped.  I can reproduce the error using
the following:

     1. Create an LVM logical volume on top of a cciss device (note that
        the cciss driver does not support barriers)
     2. Create an ext3 file system on top of the logical volume
     3. Mount the file-system using -obarrier=1
     4. Copy some files onto the file-system
     5. Run sync (dm_flush is called)

If I apply the patch (with some munging), the 

"end_request: I/O error, dev cciss/cXdY, sector 0"

messages go away.

This is not strictly a regression, given that the problem was introduced
in 2.6.31, but I think it should still be fixed for 2.6.32.


Andrew Patterson

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