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Hi Folks,
This question seems to come up from time to time and although it's
easily possible with the current tools the steps aren't obvious to everyone.
There are a number of reasons why people want to do this; accessing
SAN-based snapshots of LVM PVs, working with system images from
virtualised hosts etc.
I wrote some notes on how to do this using pvchange/vgchange/vgrename
which you'll find below. I'll add this to the wiki but if there are any
suggestions/corrections let me know & I'll include those before I set up
the page.
Regards,
Bryn.
Cloning LVM2 Volume Groups
==========================
These instructions describe the steps required to clone an LVM2 volume
group by creating a duplicate copy of the physical storage (PVs). This
requires the VG be deactivated while the clone is created and
re-named.
Linux loopback devices (/dev/loopN) are used in this example and
provide a convenient method for testing. The volume group being
cloned, t0 consists of two PVs originally present on /dev/loop0 and
/dev/loop1. A new volume group named t0-clone will be created on
devices /dev/loop2 and /dev/loop3.
1. Deactivate the VG
# vgchange -an t0
2. Create the cloned PV(s)
E.g., dd, clone LUNs on storage, break mirror etc.
# dd if=/dev/loop0 of=/dev/loop2
# dd if=/dev/loop1 of=/dev/loop3
3. For each original PV, create a filter entry in /etc/lvm/lvm.conf to
temporarily mask the PV from the LVM tools.
Preserve a copy of the original filtering rules so that it can be
restored at the end of the process, for example:
# cp /etc/lvm/lvm.conf /etc/lvm/lvm.conf.orig
To exclude the original loopback devices /dev/loop0 and /dev/loop1, the
filter line could look like this:
filter = [ "r|/dev/loop0|", "r|/dev/loop1|", "a|.*|" ]
Or, using a regex to match both devices with a single rule:
filter = [ "r|/dev/loop[01]|", "a|.*|" ]
Once the filters are set up, remove the LVM persistent cache:
# rm -f /etc/lvm/.cache [versions before 2.02.23]
OR
# rm -f /etc/lvm/cache/.cache [version 2.02.23 or later]
Verify that the filtering is correct by running pvscan:
# pvscan
PV /dev/loop2 VG t0 lvm2 [60.00 MB / 40.00 MB free]
PV /dev/loop3 VG t0 lvm2 [60.00 MB / 40.00 MB free]
Total: 2 [120.00 MB] / in use: 2 [120.00 MB] / in no VG: 0 [0 ]
Only the cloned PVs should be displayed. If the original PVs appear,
check the syntax of the filtering rule and clear the persistent cache
again.
4. Modify the cloned volume group name, ID and physical volume IDs to
avoid name and UUID clashes between the original and cloned devices:
For each cloned physical volume, run:
# pvchange --uuid /path/to/physical/volume
This will generate a new random UUID for the specified physical volume
and update the volume group metadata to reflect the changed identity.
For example:
# pvchange --uuid /dev/loop2
Physical volume "/dev/loop2" changed
1 physical volume changed / 0 physical volumes not changed
# pvchange --uuid /dev/loop3
Physical volume "/dev/loop3" changed
1 physical volume changed / 0 physical volumes not changed
Generate a new UUID for the entire volume group using vgchange:
# vgchange --uuid t0
Volume group "t0" successfully changed
Finally, rename the cloned VG:
# vgrename t0 t0-clone
5. Remove filtering rules & verify both VGs co-exist correctly
Restore the original filtering configuration and wipe the persistent cache:
# cp /etc/lvm/lvm.conf.orig /etc/lvm/lvm.conf
cp: overwrite `/etc/lvm/lvm.conf'? y
# rm -f /etc/lvm/.cache
Run pvscan to verify the new and old VGs are correctly displayed:
# pvscan
PV /dev/loop0 VG t0 lvm2 [60.00 MB / 40.00 MB free]
PV /dev/loop1 VG t0 lvm2 [60.00 MB / 40.00 MB free]
PV /dev/loop2 VG t0-clone lvm2 [60.00 MB / 40.00 MB free]
PV /dev/loop3 VG t0-clone lvm2 [60.00 MB / 40.00 MB free]
Total: 4 [240.00 MB] / in use: 4 [240.00 MB] / in no VG: 0 [0 ]
6. Activate volume groups
Both the original and cloned VGs can now be activated simultaneously:
# vgchange -ay t0
1 logical volume(s) in volume group "t0" now active
# vgchange -ay t0-clone
1 logical volume(s) in volume group "t0-clone" now active
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