Re: convert dm-thin to use dm-bufio

[Mike Snitzer <snitzer@redhat.com>]

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.
 
> An alternative would be to iterate through all the pools in the system
> check whether any of them already had the same metadata device open.
> Of course this doesn't catch the cases where the stacking is used and
> a metadata device eventually maps onto the same physical disk as an
> existing md area.

The lack of checking for a metadata or data device that is already in
use should probably be fixed.

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.