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Re: [linux-lvm] Snapshots on clustered LVM



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


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