[dm-devel] Re: [git pull] device-mapper update for 2.6.25

[Alasdair G Kergon]

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 at redhat.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 at redhat.com