Re: [linux-lvm] Snapshots on clustered LVM

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 25.8.2015 v 12:09 Bram Klein Gunnewiek napsal(a):
> 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?

Yes - snapshots are supported ONLY for exclusively actived volumes (means LV 
with snapshot is active only on a single node in cluster).

There is no dm target which would support clustered usage of snapshots.

Zdenek