mjhjmtu

Is it possible to move a logical volume from one volume group to another in whole?

It is possible to create a (more or less) matching lv and copy the data over, but is there any way to do this with LVM tools alone?

If not, is there a theoretical reason or a technical limitation (extent sizes)?

A volume group consists of whole physical volumes. A physical volume consists of many extents (an extent is typically 4MB); each extent may belong to a different logical volume. To transfer a logical volume to a different group, you cannot simply transfer extents, because that might split the physical volume between the source VG and the target VG.

What you can do is transfer one or more PVs from the source VG to the target VG, with the vgsplit command. You can specify which PVs you want to transfer, or which LV (but only one at a time). If you specify an LV, it and the other LVs in the source VG must be on separate PVs. The destination VG will be created if no VG exists with the specified name.

vgsplit -n source_group/volume_to_copy source_group target_group
vgsplit source_group target_group /dev/sdy99 /dev/sdz99

You may need to use pvmove first to arrange for the logical volumes you want to move to be on separate PVs.

If you meant to retain the physical boundaries of the VG and move the data, there's no built-in tool, but you could make a mirror then remove the original.

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 vg1 into vg2.

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
dest-pv argument tells lvconvert to make the mirror 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/lv1 inactive.

lvchange -a n /dev/vg2/lv1

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 volumes in vg2 belonging to vg1 must be inactive. In our case, this is lv1.

I will offer my own:

umount /somedir/

lvdisplay /dev/vgsource/lv0 --units b

lvcreate -L 12345b -n lv0 vgtarget

dd if=/dev/vgsource/lv0 of=/dev/vgtarget/lv0 bs=1024K conv=noerror,sync status=progress

mount /dev/vgtarget/lv0 /somedir/

if everything is good, remove the source

lvremove vgsource/lv0