From mboxrd@z Thu Jan  1 00:00:00 1970
From: Phillip Susi <psusi@cfl.rr.com>
Subject: Re: Error target causes entire device to error
Date: Thu, 13 Nov 2008 14:43:02 -0500
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References: <4918B2BC.8020606@cfl.rr.com> <491C743A.9060609@redhat.com>
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Milan Broz wrote:
> it seems that someone optimize the request, direct io does what you want.
> in fact, dm receives bio requesting 1024 bytes, not 512.
> (optimizing it to page size, but here it is limited by device size).
> 
> assuming that your arch has 4kb pages, try to setup device with
> 0 8 zero
> 8 8 error

Ahh, of course!  The buffer cache is trying to read a whole page.  And I 
guess it just gives up and fails the entire request rather than retrying 
only on the requested area.