[linux-lvm] blkid_wiping breaks snapshots

[Zdenek Kabelac]

Dne 10.2.2014 15:37, Christian Hesse napsal(a):
> 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
>


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.

Current logic is somewhat overcomplicated.

Zdenek