[linux-lvm] blkid_wiping breaks snapshots

[Christian Hesse]

Peter Rajnoha <prajnoha at redhat.com> on Mon, 2014/02/10 15:12:
> On 02/10/2014 02:48 PM, Christian Hesse wrote:
> > Zdenek Kabelac <zkabelac at redhat.com> on Mon, 2014/02/10 13:20:
> >> Dne 10.2.2014 10:30, Christian Hesse napsal(a):
> >>> Zdenek Kabelac <zkabelac at redhat.com> on Mon, 2014/02/10 09:33:
> >>>> Dne 8.2.2014 00:55, Christian Hesse napsal(a):
> >>>>> Christian Hesse <list at eworm.de> on Sat, 2014/02/08 00:36:
> >>>>>> Hello everybody,
> >>>>>>
> >>>>>> I think I nailed it down with git bisect. My first bad commit is:
> >>>>>>
> >>>>>> 75628f341ad38b68aae33eae0b5700be2a6e5769
> >>>>>> configure: enable blkid_wiping by default if the blkid library is
> >>>>>> present
> >>>>>>
> >>>>>> Looks like this wipes data that is still needed... Building a package
> >>>>>> with '--disable-blkid_wipe' now to verify on another system.
> >>>>>
> >>>>> Uh, this only helps part of...
> >>>>>
> >>>>> I changed my test setup and used writable snapshots. After that I got:
> >>>>>
> >>>>> WARNING: DM_snapshot_cow signature detected on /dev/cvg/snap-home at
> >>>>> offset 0. Wipe it? [y/n]
> >>>>>
> >>>>> (see https://bbs.archlinux.org/viewtopic.php?id=176504 for another
> >>>>> report)
> >>>>>
> >>>>> Looks like disabling blkid_wiping fixes this. My snapshot corruption
> >>>>> still occurs though. :-/
> >>>>> Bad thing about it is that the corruption does not occur reliable when
> >>>>> doing simple tests in 'git bisect'... Out of ideas for now - Will go
> >>>>> to bed now.
> >>>>
> >>>> Now this was helpful, I guess I think what is going on.
> >>>
> >>> Great! Waiting for patches then. :D
> >>>
> >>> I could provide some more information when needed:
> >>>
> >>> * This happens with read only snapshots only.
> >>> * I could reproduce with commit eaa23d32732c9bc3dd4f948781b5764cf21d84ba
> >>>    (wiping: add support for blkid wiping), so it was introduced there or
> >>>    before.
> >>>
> >>> Should I investigate further or wait for something to test from you?
> >>
> >> Is the patch bellow fixing your problem ?
> >> (It's still not final - but should help)
> >>
> >> Zdenek
> >>
> >>
> >> diff --git a/tools/lvcreate.c b/tools/lvcreate.c
> >> index 638a868..e8b1a7f 100644
> >> --- a/tools/lvcreate.c
> >> +++ b/tools/lvcreate.c
> >> @@ -772,7 +772,7 @@ static int _read_activation_params(struct
> >> lvcreate_params *lp, LVM_READ | LVM_WRITE);
> >>
> >>          /* Must not zero/wipe read only volume */
> >> -       if (!(lp->permission & LVM_WRITE)) {
> >> +       if (!lp->snapshot && !(lp->permission & LVM_WRITE)) {
> >>                  lp->zero = 0;
> >>                  lp->wipe_signatures = 0;
> >>          }
> > 
> > This looks good to me. I've made some backups now without any corruption.
> > 
> > But with blkid_wiping enabled I do have this warning/question now for read
> > only snapshots as well:
> > 
> > WARNING: DM_snapshot_cow signature detected on /dev/cvg/snap-home
> > at offset 0. Wipe it? [y/n]
> > 
> > lvm works perfectly only with your patch applied and compiled with
> > --disable-blkid_wiping.
> 
> Also, try this patch in addition:
> 
> https://git.fedorahosted.org/cgit/lvm2.git/commit/?id=ed166a3b1d3290ad887d8f83c24a8d8877713d3c

That did the trick. Thanks a lot!

If anybody is interested... These are the patches updated to apply against
version 2.02.105:

http://www.eworm.de/download/linux/lvm2-snapshot.patch
http://www.eworm.de/download/linux/lvm2-snapshot-wiping.patch
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