linux-kernel - Re: [git pull] device-mapper update for 2.6.25

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Message-ID: <20080208032423.GX8075@agk.fab.redhat.com>
Date:	Fri, 8 Feb 2008 03:24:23 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Wood <brian.j.wood@...el.com>,
	Daniel Walker <dwalker@...sta.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jonathan Brassow <jbrassow@...hat.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	Milan Broz <mbroz@...hat.com>, Paul Jimenez <pj@...ce.org>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Vasily Averin <vvs@...ru>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [git pull] device-mapper update for 2.6.25

On Thu, Feb 07, 2008 at 07:04:47PM -0800, Andrew Morton wrote:
> On Fri, 8 Feb 2008 02:45:32 +0000 Alasdair G Kergon <agk@...hat.com> wrote:
> 
> >       dm: move deferred bio flushing to workqueue
> 
> To my unpracticed eye this one looks fairly fundamental.  Unfortunately the
> changelog:
> 
>   Add a single-thread workqueue for each mapped device and move flushing
>   of the lists of pushback and deferred bios to this new workqueue.
> 
> tells us nothing of the reasons for this change.  (It should have done so!)
> 
> It _looks_ pretty unscalable.  Perhaps these deferred BIOs don't happen
> very often?
 
Indeed - it's a rare occurrence - happens when the dm device is
suspended/resumed which usually means it was being reconfigured by something in
userspace.

It's the first patch of a series that will provide barrier support (slated for
2.6.26).

The appearance of all these workqueues in dm goes back to this old change:
  md-dm-reduce-stack-usage-with-stacked-block-devices.patch

Alasdair
-- 
agk@...hat.com
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