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<p style="margin-bottom: 0in; line-height: 100%">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.
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<p style="margin-bottom: 0in; line-height: 100%">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.
</p>
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<p style="margin-bottom: 0in; line-height: 100%">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).
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<p style="margin-bottom: 0in; line-height: 100%">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:
<a class="moz-txt-link-freetext" href="https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes">https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html-single/Logical_Volume_Manager_Administration/#snapshot_volumes</a>).
</p>
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<p style="margin-bottom: 0in; line-height: 100%">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.
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<p style="margin-bottom: 0in; line-height: 100%">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?</p>
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<pre class="moz-signature" cols="72">--
Met vriendelijke groet / Kind regards,
Bram Klein Gunnewiek | Shock Media B.V.
Tel: +31 (0)546 - 714360
Fax: +31 (0)546 - 714361
Web: <a class="moz-txt-link-freetext" href="https://www.shockmedia.nl/">https://www.shockmedia.nl/</a></pre>
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