[linux-lvm] blkid_wiping breaks snapshots

[Christian Hesse]

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.
-- 
main(a){char*c=/*    Schoene Gruesse                         */"B?IJj;MEH"
"CX:;",b;for(a/*    Chris           get my mail address:    */=0;b=c[a++];)
putchar(b-1/(/*               gcc -o sig sig.c && ./sig    */b/42*2-3)*42);}
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