From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Thornber <thornber@redhat.com>
Subject: Re: convert dm-thin to use dm-bufio
Date: Fri, 19 Aug 2011 11:22:16 +0100
Message-ID: <20110819102215.GA5069@ubuntu>
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To: Mike Snitzer <snitzer@redhat.com>
Cc: dm-devel@redhat.com, Mikulas Patocka <mpatocka@redhat.com>, "Alasdair G. Kergon" <agk@redhat.com>
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On Fri, Aug 19, 2011 at 05:46:04AM -0400, Mike Snitzer wrote:
> On Fri, Aug 19 2011 at  5:11am -0400,
> Joe Thornber <thornber@redhat.com> wrote:
> 
> > On Fri, Aug 19, 2011 at 03:04:36AM -0400, Mike Snitzer wrote:
> > > Question for Joe:
> > > You're making conflicting changes quick enough that I wonder if you
> > > and Mikulas will ever converge (e.g. why do multiple block managers need
> > > to have access to the same metadata device!?).
> > 
> > They don't; my issue is with getting an oops if they do through user
> > error.  I clearly said in the commit message that this was a hack to
> > get round issues introduced by agk's move to a kmemcache.  Cook
> > something cleaner up between yourselves, or wait for me to look at it
> > again once I've got through some more pressing issues.
> 
> OK, so this kmemcache problem will go away once you switch over to
> bufio.

Assuming agk doesn't decide to switch over to a kmemcache.  Remember
bm used to allocate via kmalloc or pages like bufio.

> As for stacking, can't we just read the superblock to check if a device
> is already in use (would work metadata anyway)?  No idea if that'd be
> too costly -- probably not: read superblock and check if
> THIN_SUPERBLOCK_MAGIC is set.

So you've already thought of a scenario where it's worth having two bm
instances (one read only).

- Joe