Re: [linux-lvm] Creating/restoring snapshots in early userspace

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

Dne 08. 08. 22 v 20:14 cd napsal(a):
>>
>>
>> Dne 07. 08. 22 v 22:38 cd napsal(a):
>>
>>> Hello,
>>>
>>> I have created some scripts which runs in the initramfs during the boot process. Very specifically, it's an initcpio runtime hook https://man.archlinux.org/man/mkinitcpio.8#ABOUT_RUNTIME_HOOKS
>>> Question: Is this a supported environment which to create/restore snapshots?
>>>
>>> When my script runs lvm lvconvert --merge testvg/lvmautosnap-root-1659902622-good
>>> it appears to succeed (exit code is 0, and the restore appears to work properly). However, the following warnings appear in stderr as part of the restore process:
>>>
>>> /usr/bin/dmeventd: stat failed: No such file or directory
>>> WARNING: Failed to unmonitor testvg/lvmautosnap-root-1659902622-good.
>>> /usr/bin/dmeventd: stat failed: No such file or directory
>>
>>
>> Hi
>>
>> Your initramfs likely needs to contain 'modified' version of your system's
>> lvm.conf where 'monitoring' will be disabled (set to 0) - as you do not want
>> to start your monitoring while you are operating in your ramdisk.
>>
>> Once you flip to your rootfs with your regular /etc/lvm/lvm.conf - you need
>> to start monitoring of you activated LVs (vgchange --monitor y)
>>
>>> Merging of volume testvg/lvmautosnap-root-1659902622-good started.
>>> /run/lvm/lvmpolld.socket: connect failed: No such file or directory
>>
>>
>> Again a thing you do not want to run in your ramdisk - lvmpolld is another
>> service/daemon you should run while you are in your rootfs.
>>
>> fully removed.
>>
>>> And I get similar errors when trying to create new volumes with lvm lvcreate --permission=r --snapshot --monitor n --name my_snapshot
>>> /usr/bin/dmeventd: stat failed: No such file or directory
>>>
>>> In summary, I'm happy to just ignore the warning messages. I just want to make sure I'm not risking the integrity of the lvm volumes by modifying them during this part of the boot process.
>>
>>
>> It looks like you are trying to do something in your ramdisk you really should
>> be doing once you flip to your rootfs - ramdisk is purely meant to be used
>> to get things 'booting' and flip to rootfs ASAP - doing things in your
>> ramdisk which is really not a 'working environment' sounds like you are
>> asking yourself for some big troubles with resolving error paths (i.e. using
>> unmonitored devices like 'snapshot/mirror/raids/thin...' for longer period of
>> time is simply 'bad design/plan' - switch to rootfs should happen quickly
>> after you initiate things in your initramdfs...
>>
>> Regards
>>
>> Zdenek
> 
> Thanks for the insightful response. Indeed, setting monitoring = 0 in lvm.conf makes the warning messages go away. Interestingly, on arch, the initcpio hook for lvm2 _does_ attempt to set this setting:
> with sed -i '/^\smonitoring =/s/1/0/' "${BUILDROOT}/etc/lvm/lvm.conf"
> https://github.com/archlinux/svntogit-packages/blob/packages/lvm2/trunk/lvm2_install#L38
> 
> However, the sed pattern fails to match because the line is commented out in lvm.conf.
> I've filed a bug with arch to address this: https://bugs.archlinux.org/task/75552?project=1&string=lvm2
> 

Yep - I think there was a similar issue with Dracut.
It's the side-effect result of making most of default settings as 'commented' 
- then this scripts stopped to work in such case.

Regards

Zdenek

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