[linux-lvm] snapshot looses origin when other snapshot is merged to the same origin

[kostrzewa at 9livesdata.com]

On 28.09.2018 18:59, Chris Murphy wrote:
> On Fri, Sep 28, 2018 at 6:57 AM, Zbigniew Kostrzewa
> <kostrzewa at 9livesdata.com> wrote:
>> Hi,
>> 
>> I have a question related to multiple snapshots of the same thin 
>> volume. My
>> scenario is that I create a thin pool, a thin volume and two snapshots 
>> of
>> that thin volume. Both snapshots have the thin volume set as the 
>> origin
>> volume. However, after I merge one of the snapshots to the origin then 
>> the
>> second snapshot looses information about the origin.
>> 
>> The question is if that is an expected behavior or if I am doing 
>> something
>> wrong? Is there a way to make the second snapshot keep having the 
>> origin
>> with merged first snapshot as its origin?
> 
> What file system are you using? And what lvm commands are you using
> for all of this?
> 
> Someone else will have to answer the merging question, as it's not
> something I've ever used or expect to use with thin volumes. Merging,
> to me, sounds like a thick volume snapshots convention.
> 
> With thin volumes, each snapshot, even though it initially points to
> an origin, is it's own completely independent volume. Given a thin
> volume A, and snapshots created with 'lvcreate -s vg/A -n B' and
> 'lvcreate -s vg/A -n C' my experience is that A B C are initially
> identical, and modifying A does not change B or C. Modifying B does
> not change A or C. Modifying C does not change A or B.
> 
> By default, thin volume snapshots are not active volumes. So they're
> not mountable. And once two or more are active, there's the potential
> for confusion because you have literally two file systems that are
> identical as far as the kernel is concerned. They have identical
> volume UUIDs. I know XFS will refuse to mount a 2nd volume with the
> same UUID as an already mounted file system; and while this can be
> inhibited at mount time, I do not know the consequences.

Thanks for taking time to answer my question.

I don't do anything fancier than what is described in documentation for 
creating thin volumes and snapshots of thinly provisioned volumes, i.e.:

lvcreate -L30G -T vg/root-pool
lvcreate -V20G -T vg/root-pool -n root
lvcreate --snapshot -n snap1 vg/root
lvcreate --snapshot -n snap2 vg/root
lvconvert --merge snap2

My use case is like so: I have a thinly provisioned root volume (with 
rootfs on XFS). I create a snapshot after fresh installation. I install 
some software. The I want to upgrade the software but in order to be 
able to rollback the upgrade I create a second snapshot (before the 
upgrade). When I do a rollback of the upgrade (lvconvert --merge snap2 
&& reboot) I loose possibility to rollback to the first snapshot - taken 
right after fresh installation. Most probably I can simply replace my 
root volume with the first snapshot and probably I should get what I 
want, that is rootfs after fresh installation, but running lvconvert 
--merge is more convenient than replacing the origin with the snapshot 
by myself.

Regards,
Zbigniew Kostrzewa