[linux-lvm] Could we manually suspend the origin before taking dm-thin snapshots?

[linux-lvm] Could we manually suspend the origin before taking dm-thin snapshots?

[M.H. Tsai]

Hi All,

I found an issue when taking dm-thin snapshots: LVM doesn't send
create_snap messages if the origin was already suspended, and LVM
doesn't report any error.

lvcreate vg1 --type thin --thinpool tp1 --virtualsize 1g
dmsetup suspend vg1-lv1
lvcreate vg1/lv1 --snapshot -an

This is due to commit a900d150 moves messaging from resume to suspend,
then LVM skips the suspend phase in this case. I'm not sure whether it
is a legal operation for LVM, but if LVM supports this feature, then
users can control the suspend timing.


Thanks,
Ming-Hung Tsai

Re: [linux-lvm] Could we manually suspend the origin before taking dm-thin snapshots?

[Zdenek Kabelac]

Dne 31.12.2015 v 09:40 M.H. Tsai napsal(a):
> Hi All,
>
> I found an issue when taking dm-thin snapshots: LVM doesn't send
> create_snap messages if the origin was already suspended, and LVM
> doesn't report any error.
>
> lvcreate vg1 --type thin --thinpool tp1 --virtualsize 1g
> dmsetup suspend vg1-lv1
> lvcreate vg1/lv1 --snapshot -an
>

Hi

This is completely UNSUPPORTED  - you MAY NOT interfere with
LV volumes with 'dmsetup'.

You can use dmsetup only if you know EXACTLY what's going on.
The only recommended usage of dmsetup with lvm2 is - when
lvm2 lacks some error path (e.g. fix mess left after
failing lvm command).

Otherwise there is no point to hijack LVs in the middle
of LV operations - i.e. in cluster world  state of LV
(active/suspend) is tightly related to lock state.

> This is due to commit a900d150 moves messaging from resume to suspend,
> then LVM skips the suspend phase in this case. I'm not sure whether it
> is a legal operation for LVM, but if LVM supports this feature, then
> users can control the suspend timing.

plain unsupported state.

You could have been hijacking suspend/resume/remove operation
anytime. In fact you could easily block/freeze whole LV but doing
random suspends of DM devices in your system.

Regards

Zdenek