Re: [linux-lvm] Missing error handling in lv_snapshot_remove

[Andreas Pflug <pgadmin@pse-consulting.de>]

Am 08.08.13 12:01, schrieb Zdenek Kabelac:
> Dne 7.8.2013 19:18, Andreas Pflug napsal(a):
>> On 08/07/13 11:41, Zdenek Kabelac wrote:
>>> Dne 7.8.2013 11:22, Andreas Pflug napsal(a):
>>>> Am 06.08.13 19:37, schrieb Bastian Blank:
>>>>> Hi
>>>>>
>>>>> I tried to tackle a particular bug that shows up in Debian for
>>>>> some time
>>>>> now. Some blamed the udev rules and I still can't completely rule
>>>>> them
>>>>> out. But this triggers a much worse bug in the error cleanup of the
>>>>> snapshot remove. I reproduced this with Debian/Linux 3.2.46/LVM
>>>>> 2.02.99
>>>>> without udevd running and Fedora 19/LVM 2.02.98-10.fc19.
>>>>>
>>>>> On snapshot removal, LVM first converts the device into a regular LV
>>>>> (lv_remove_snapshot) and in a second step removes this LV
>>>>> (lv_remove_single). Is there a reason for this two step removal? An
>>>>> error during removal leaves a non-snapshot LV behind.
>>>> Ah, this explains why sometimes my backup stops: I take a snapshot,
>>>> rsync the stuff and remove the snapshot with a daily cron job, but I
>>>> observed twice that a non-snapshot volume named like a backup snapshot
>>>> was lingering around, preventing the script to work. So this is no
>>>> exotic corner case, but happens in real life.
>>>>
>>>> I observe this since I dist-upgraded to wheezy.
>>>>
>>>
>>> Because Debian is using non-upstream udev rules.
>>>
>>> With upstream udev rules with standard real-life use, this situation
>>> cannot happen - since these rules are constructed to play better with
>>> udev WATCH rule.
>>
>> Hm, does udev play a role on this at all? Without having dived the
>> code, I'd
>> assume udev has only to do with creation and deletion of /dev/mapper/...
>> and/or /dev/vgname/... devices (upon lvchange -aX), but not with lvm
>> metadata
>> manipulation.
>
>
> Udev attempts to update it device database after any change event
> (you could observe its work with udevadm monitor)
>
> So in your case -  you unmount filesystem -> close device -> fires
> WATCH event with some randomly delayed (systemd)udevd scan machism -
> so in unpredictable moment blkid opens device and scans its sectors
> (keeping device open and interfering with deactivate operation). For
> this short-time opens there is now built-in retry which tries to
> deactivate device several times when it's known device is not mounted.

So in order to harden my script against this problem, I should
deactivate the volume explicitely, wait a while and then remove it?

Regards,
Andreas