On 08/26/2015 12:59 PM, Zdenek Kabelac wrote:
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
Thanks for the confirmation. It's a pitty we can't get this done with
LVM ... we will try to find an alternative.
Out of curiosity, how does a node know the volume is opened at another
node? In our test set-up we don't use CLVM or anything (we are just
testing), so there is no communication between the nodes. Is this done
through meta data in the volume group / logical volume?
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