On 28.09.2018 18:59, Chris Murphy wrote:
On Fri, Sep 28, 2018 at 6:57 AM, Zbigniew Kostrzewa
<kostrzewa@xxxxxxxxxxxxxx> 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
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