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From: Zdenek Kabelac <zkabelac@redhat.com>
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Date: Thu, 31 Dec 2015 22:12:47 +0100
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Subject: Re: [linux-lvm] Could we manually suspend the origin before taking
 dm-thin snapshots?
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To: LVM general discussion and development <linux-lvm@redhat.com>

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