linux-kernel - Re: [dm-devel] dm: dm-cache fails to write the cache device in writethrough mode

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Date:	Sat, 23 Mar 2013 03:48:03 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	Mike Snitzer <snitzer@...hat.com>,
	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Heinz Mauelshagen <heinzm@...hat.com>,
	linux-kernel@...r.kernel.org,
	device-mapper development <dm-devel@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Joe Thornber <ejt@...hat.com>
Subject: Re: [dm-devel] dm: dm-cache fails to write the cache device in
	writethrough mode

On Fri, Mar 22, 2013 at 11:27:16PM -0400, Mike Snitzer wrote:
> So the big difference is the thinp-test-suite uses intermediate linear
> DM layers above the slower sd device (spindle/data) -- whereas in your
> setup the origin device is direct to sd (8:0).
 
I would think the difference is because DM issues (edited) copies of
incoming bios to the layers underneath, so changes don't propagate back
up.

Darrick: if you retest with a dm linear device, does the problem go
away?

Alasdair

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