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Re: [linux-lvm] blkid_wiping breaks snapshots



Dne 10.2.2014 19:15, Peter Rajnoha napsal(a):
On 02/10/2014 04:56 PM, Zdenek Kabelac wrote:
Dne 10.2.2014 15:37, Christian Hesse napsal(a):
Peter Rajnoha <prajnoha redhat com> on Mon, 2014/02/10 15:12:
On 02/10/2014 02:48 PM, Christian Hesse wrote:
Zdenek Kabelac <zkabelac redhat com> on Mon, 2014/02/10 13:20:
Dne 10.2.2014 10:30, Christian Hesse napsal(a):
Zdenek Kabelac <zkabelac redhat com> on Mon, 2014/02/10 09:33:
Dne 8.2.2014 00:55, Christian Hesse napsal(a):
Christian Hesse <list 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



We need to resolve the logic behind the query for wiping. We will most
probably go with this logic:

We query for wipe of 'known' signatures, before passing newly allocated
'normal' LV (visible).
Any other so called 'private/hidden' LV will do either wipe or
unconditional zeroing of 1st. 4kb - which is usually the only thing
needed here.


MD can write its signature beyond 4K. Just an example...

Which is exactly the thing which should not matter for case like '-cow'

Since this LV is 'private' and nobody should ever touch/use/read this device
except snapshot target.

By removing all signatures could actually mask bugs in i.e. udev rules,
where  md rules would read our own private device.

Zdenek.


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