[linux-lvm] Snapshots on clustered LVM

[Bram Klein Gunnewiek]

Currently we are using LVM as backing storage for our DRBD disks in HA 
set-ups. We use QEMU instances on our node's using (local) DRBD targets 
for storage. This enables us to do live migrations between the DRBD 
primary/secondary nodes.

We want to support iSCSI targergets in our HA enviroment. We are trying 
to see if we can use (c)lvm for that by creating a volume group of our 
iSCSI block devices and use that volume group on all nodes to create 
logical volumes. This seems to work fine if we handle locking etc 
properly and make sure we only activate the logical volumes on one node 
at a time. As long as we only have a volume active on one node snapshots 
seem to work fine also.

However, we run into problems when we want to perform a live migration 
of a running QEMU instance. In order to do a live migration we have to 
start a second similar QEMU on the node we want to migrate to and start 
a QEMU live migration. In order for us to do that we have to make the 
logical volume active on the target node otherwise we can't start the 
QEMU instance. During the live migration QEMU ensures that data is only 
written on one node (e.g. during the live migration data will be written 
on the source node, QEMU wil then pause the instance for a short while 
when copying the last data and will then continue the instance on the 
target node).

This use case works fine with a clustered LVM set-up except for 
snapshots. Changes are not saved in the snapshot when the logical volume 
is active on both nodes (as expected if the manual is correct: 
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes). 


If we are correct it means we can use lvm for as clustered "file system" 
but can't trust our snapshots to be 100% reliable if a volume group has 
been made active on more then one node. E.G. when doing a live migration 
between two nodes of a QEMU instance our snapshots become unreliable.

Are these conclusions correct? Is there a solution for this problem or 
is this simply a known limitation of clustered lvm without a work-around?

-- 
Met vriendelijke groet / Kind regards,
Bram Klein Gunnewiek | Shock Media B.V.

Tel: +31 (0)546 - 714360
Fax: +31 (0)546 - 714361
Web: https://www.shockmedia.nl/

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