[lvm-devel] a bug in snapshots

[Mikulas Patocka]

On Tue, 9 Feb 2010, Mike Snitzer wrote:

> On Tue, Feb 09 2010 at  7:07pm -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > Hi
> > 
> > Try to make a snapshot that is so big that it spans more than one physical 
> > volume. Then, type "lvs" command. You get these errors:
> > 
> > [slunicko:/usr/src/LVM2.2.02.60]# lvs
> >   Number of segments in active LV lvol1 does not match metadata
> >   Number of segments in active LV lvol1 does not match metadata
> >   Number of segments in active LV lvol1 does not match metadata
> >   Number of segments in active LV lvol1 does not match metadata
> >   LV    VG   Attr   LSize  Origin Snap%  Move Log Copy%  Convert
> >   lvol0 vg1  owi-a- 16.00m
> >   lvol1 vg1  Swi-I- 60.00m lvol0  100.00
> >   m     vg1  -wi-a- 64.00m
> > 
> > This bug was introduced in LVM2.2.02.59 with this change:
> > 
> > --- ./LVM2.2.02.58/lib/activate/dev_manager.c   2010-01-13 
> > 02:55:44.000000000 +0100
> > +++ ./LVM2.2.02.59/lib/activate/dev_manager.c   2010-01-15 
> > 23:58:25.000000000 +0100
> > @@ -584,7 +593,7 @@ int dev_manager_snapshot_percent(struct
> >          * Try and get some info on this device.
> >          */
> >         log_debug("Getting device status percentage for %s", name);
> > -       if (!(_percent(dm, name, dlid, "snapshot", 0, NULL, percent,
> > +       if (!(_percent(dm, name, dlid, "snapshot", 0, lv, percent,
> >                        percent_range, NULL)))
> >                 return_0;
> > 
> 
> That change was part of this commit:
> http://sources.redhat.com/git/?p=lvm2.git;a=commit;h=13713bc147
> 
> I'm not seeing how this commit has any functional change for anything
> other than snapshot-merge related snapshots.
> 
> All I did was pass lv into _percent() (and then _percent_run()) so that
> I could inspect it to know if lv_is_merging_origin().
> 
> The vg1/lvol1's flags indicate the snapshot is set to merge and it is
> also invalid.  Can you detail your testcase a bit more please?
> 
> Are you certain there isn't some other reason your test case is failing?
> 
> Mike

The problem is that when you pass non-NULL lv to _percent_run(), it tries 
to verify that the number of segments in that lv is equal to the number of 
targets. And it isn't --- the lv can have more than one segment but there 
is just one "snapshot" target. That's why it fails.

What to do with it? Drop this verification? Or pass NULL as lv there, like 
before?

Mikulas