Re: [Qemu-devel] [RFC V6 27/33] qcow2: Adapt checking of QCOW_OFLAG_COPIED for dedup.

[Kevin Wolf <kwolf@redhat.com>]

Am 28.02.2013 um 17:14 hat Benoît Canet geschrieben:
> Le Thursday 28 Feb 2013 à 11:14:34 (+0100), Kevin Wolf a écrit :
> > Am 28.02.2013 um 10:41 hat Stefan Hajnoczi geschrieben:
> > > On Wed, Feb 27, 2013 at 04:00:28PM +0100, Benoît Canet wrote:
> > > > > > -                if ((refcount == 1) != ((l2_entry & QCOW_OFLAG_COPIED) != 0)) {
> > > > > > +                if (!s->has_dedup &&
> > > > > > +                    (refcount == 1) != ((l2_entry & QCOW_OFLAG_COPIED) != 0)) {
> > > > > > +                    fprintf(stderr, "ERROR OFLAG_COPIED: offset=%"
> > > > > > +                        PRIx64 " refcount=%d\n", l2_entry, refcount);
> > > > > > +                    res->corruptions++;
> > > > > > +                }
> > > > > 
> > > > > Why is this warning suppressed when dedup is enabled?  The meaning of
> > > > > QCOW_OFLAG_COPIED is that refcount == 1.  If this invariant is violated
> > > > > then something is wrong.
> > > > 
> > > > When deduplication is done refcount will be bigger than one and
> > > > QCOW_OFLAG_COPIED will be cleared.
> > > > 
> > > > Then if enough logical clustere pointing to the same physical cluster are
> > > > rewritten with something else the refcount will goes down back to one.
> > > > 
> > > > But this time QCOW_OFLAG_COPIED can be set back so this equality won't be true.
> > > 
> > > When the refcount decreases to 1 again we need to set QCOW_OFLAG_COPIED
> > > again.  qcow2-snapshot.c:qcow2_snapshot_delete() does this with:
> > > 
> > >     /* must update the copied flag on the current cluster offsets */
> > >     ret = qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
> > > 
> > > Is dedup not restoring QCOW_OFLAG_COPIED?
> > 
> > This is a very expensive operation. I don't think that you can do it for
> > each deduplicated cluster that is overwritten. Not doing it comes with
> > the cost of doing more COW than is actually needed. And we need to
> > mention in the spec that QCOW_OFLAG_COPIED can be missing on clusters
> > with deduplication enabled.
> 
> Also when two logical clusters point to the same physical cluster and one of the
> logical cluster get overwritten the deduplication code has no way to know the
> index of the last logical cluster entry.

Well, strictly speaking you can. The qcow2_update_snapshot_refcount()
call that Stefan mention does exactly that. It's just insanely expensive
because it has to look at the refcounts for all clusters.

Kevin