[lvm-devel] a bug in snapshots
Mike Snitzer
snitzer at redhat.com
Wed Feb 10 13:40:35 UTC 2010
On Tue, Feb 09 2010 at 11:01pm -0500,
Mike Snitzer <snitzer at redhat.com> wrote:
> On Tue, Feb 09 2010 at 8:57pm -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
>
> >
> >
> > 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.
>
> I see. I was too focused on the commit; didn't look at the rest of
> _percent_run(), etc.
>
> > What to do with it? Drop this verification? Or pass NULL as lv there, like
> > before?
>
> Well the reality is we really shouldn't be passing a snapshot-origin
> into _percent_run() to begin with. I made it so that we filter it out
> very late, in _percent_run, in cases where snapshot-merge completion
> polling gets started even though the snapshot was not allowed to start
> merging (as is possible if a snapshot or snapshot-origin is still open
> when a 'lvchange --refresh origin' occurs).
>
> So one thing we could do is move the lv_is_merging_origin() much earlier
> and revert to passing a NULL lv to _percent().
>
> Another option is to audit that segment count check in _percent_run()
> and possibly only make that check for cases where it really matters.
Here is what I came up with. Justification for this approach is
explained in the dev_manager_snapshot_percent() comment below.
diff --git a/lib/activate/dev_manager.c b/lib/activate/dev_manager.c
index 6fbc392..ab4f16c 100644
--- a/lib/activate/dev_manager.c
+++ b/lib/activate/dev_manager.c
@@ -423,7 +423,7 @@ static int _percent_run(struct dev_manager *dm, const char *name,
const char *target_type, int wait,
const struct logical_volume *lv, float *percent,
percent_range_t *overall_percent_range,
- uint32_t *event_nr)
+ uint32_t *event_nr, int fail_if_percent_unsupported)
{
int r = 0;
struct dm_task *dmt;
@@ -516,10 +516,8 @@ static int _percent_run(struct dev_manager *dm, const char *name,
/* above ->target_percent() was not executed! */
/* FIXME why return PERCENT_100 et. al. in this case? */
*overall_percent_range = PERCENT_100;
- if (lv && lv_is_merging_origin(lv)) {
- /* must fail in snapshot-merge case */
+ if (fail_if_percent_unsupported)
goto_out;
- }
} else
*overall_percent_range = combined_percent_range;
}
@@ -535,20 +533,24 @@ static int _percent_run(struct dev_manager *dm, const char *name,
static int _percent(struct dev_manager *dm, const char *name, const char *dlid,
const char *target_type, int wait,
const struct logical_volume *lv, float *percent,
- percent_range_t *overall_percent_range, uint32_t *event_nr)
+ percent_range_t *overall_percent_range, uint32_t *event_nr,
+ int fail_if_percent_unsupported)
{
if (dlid && *dlid) {
if (_percent_run(dm, NULL, dlid, target_type, wait, lv, percent,
- overall_percent_range, event_nr))
+ overall_percent_range, event_nr,
+ fail_if_percent_unsupported))
return 1;
else if (_percent_run(dm, NULL, dlid + sizeof(UUID_PREFIX) - 1,
target_type, wait, lv, percent,
- overall_percent_range, event_nr))
+ overall_percent_range, event_nr,
+ fail_if_percent_unsupported))
return 1;
}
if (name && _percent_run(dm, name, NULL, target_type, wait, lv, percent,
- overall_percent_range, event_nr))
+ overall_percent_range, event_nr,
+ fail_if_percent_unsupported))
return 1;
return 0;
@@ -607,6 +609,21 @@ int dev_manager_snapshot_percent(struct dev_manager *dm,
{
char *name;
const char *dlid;
+ int fail_if_percent_unsupported = 0;
+
+ if (lv_is_merging_origin(lv)) {
+ /*
+ * Passing unsupported LV types to _percent will lead to a
+ * default successful return with percent_range as PERCENT_100.
+ * - For a merging origin, this will result in a polldaemon
+ * that runs infinitely (because completion is PERCENT_0)
+ * - We unfortunately don't yet _know_ if a snapshot-merge
+ * target is active (activation is deferred if dev is open);
+ * so we can't short-circuit origin devices based purely on
+ * existing LVM LV attributes.
+ */
+ fail_if_percent_unsupported = 1;
+ }
/*
* Build a name for the top layer.
@@ -621,8 +638,8 @@ int dev_manager_snapshot_percent(struct dev_manager *dm,
* Try and get some info on this device.
*/
log_debug("Getting device status percentage for %s", name);
- if (!(_percent(dm, name, dlid, "snapshot", 0, lv, percent,
- percent_range, NULL)))
+ if (!(_percent(dm, name, dlid, "snapshot", 0, NULL, percent,
+ percent_range, NULL, fail_if_percent_unsupported)))
return_0;
/* FIXME dm_pool_free ? */
@@ -657,7 +674,7 @@ int dev_manager_mirror_percent(struct dev_manager *dm,
log_debug("Getting device mirror status percentage for %s", name);
if (!(_percent(dm, name, dlid, "mirror", wait, lv, percent,
- percent_range, event_nr)))
+ percent_range, event_nr, 0)))
return_0;
return 1;
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