Is there a simple way to move/copy a logical volume from one volume group to another? (LVM2)

Question

I'm rearranging a bunch of disks on my server at home and I find myself in the position of wanting to move a bunch of LVM logical volumes to another volume group. Is there a simple way to do this? I saw mention of a cplv command but this seems to be either old or not something that was ever available for Linux.

Answer 1

If you can arrange for the logical volume to be on a separate subset of physical volumes from the rest of the source volume group (lvconvert sourcevg/sourcelv /dev/pv1 ... may help), you can use vgsplit to split off the lv into a new vg and vgmerge to merge the new vg into the target vg.

Although LVM has a mirroring feature, you can't (sanely) use it to make a copy between volume groups, because both legs of the mirror must live on the same vg and the association can't be broken.

You can copy an LVM volume to another the way you'd copy any volume to another: create a target lv of the appropriate size, then copy the contents with dd if=/dev/sourcevg/sourcelv of=/dev/targetvg/targetlv bs=4M. If the source volume is active, you can leverage LVM to mkae a consistent copy: first take a snapshot of the source lv with lvcreate -s, then copy the snapshot.

Answer 2

pvmove -n lvol1 /dev/sdb1 /dev/sdc1

pvmove can move data between physical volumes: LVM Administrator's Guide

Answer 3

As of the LVM in Debian stretch (9.0), namely 2.02.168-2, it's possible to do a copy of a logical volume across volume groups using a combination of vgmerge, lvconvert, and vgsplit. Since a move is a combination of a copy and a delete, this will also work for a move.

Alternatively, you can use pvmove to just move the volume.

A complete self-contained example session using loop devices and lvconvert follows.

Summary: we create volume group vg1 with logical volume lv1, and vg2 with lv2, and make a copy of lv1 in vg2.

Create files.

truncate pv1 --size 100MB
truncate pv2 --size 100MB

Set up loop devices on files.

losetup /dev/loop1 pv1
losetup /dev/loop2 pv2

Create physical volumes on loop devices (initialize loop devices for use by LVM).

pvcreate /dev/loop1 /dev/loop2

Create volume groups vg1 and vg2 on /dev/loop1 and /dev/loop2 respectively.

vgcreate vg1 /dev/loop1
vgcreate vg2 /dev/loop2

Create logical volumes lv1 and lv2 on vg1 and vg2 respectively.

lvcreate -L 10M -n lv1 vg1
lvcreate -L 10M -n lv2 vg2

Create ext4 filesystems on lv1 and lv2.

mkfs.ext4 -j /dev/vg1/lv1
mkfs.ext4 -j /dev/vg2/lv2

Optionally, write something on lv1 so you can later check the copy was correctly created. Make vg1 inactive.

vgchange -a n vg1

Run merge command in test mode. This merges lv1 into lv2.

vgmerge -A y -l -t -v <<destination-vg>> <<source-vg>>
vgmerge -A y -l -t -v vg2 vg1

And then for real.

vgmerge -A y -l -v vg2 vg1

Then create a RAID 1 mirror pair from lv1 using lvconvert. The <> argument tells lvconvert to make the mirror copy lv1_copy on /dev/loop2.

lvconvert --type raid1 --mirrors 1 <<source-lv>> <<dest-pv>>
lvconvert --type raid1 --mirrors 1 /dev/vg2/lv1 /dev/loop2

Then split the mirror. The new LV is now lv1_copy.

lvconvert --splitmirrors 1 --name <<source-lv-copy>> <<source-lv>>
lvconvert --splitmirrors 1 --name lv1_copy /dev/vg2/lv1

Make vg2 inactive.

vgchange -a n vg2

Then (testing mode)

vgsplit -t -v <<source-vg>> <<destination-vg>> <<moved-to-pv>>
vgsplit -t -v /dev/vg2 /dev/vg1 /dev/loop1

For real

vgsplit -v /dev/vg2 /dev/vg1 /dev/loop1

Resulting output:

lvs
[...]
lv1        vg1       -wi-a-----  12.00m
lv1_copy   vg2       -wi-a-----  12.00m
lv2        vg2       -wi-a-----  12.00m

NOTES:

1) Most of these commands will need to be run as root.

2) If there is any duplication of the names of the logical volumes in the two volume groups, vgmerge will refuse to proceed.

3) On merge:

Logical volumes in `vg1` must be inactive

And on split:

Logical volume `vg2/lv1` must be inactive.