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Re: [linux-lvm] resize and snapshots with clvm



Am 20.02.13 14:30, schrieb Jacek Konieczny:
On Wed, 20 Feb 2013 14:18:23 +0100
Andreas Pflug <pgadmin pse-consulting de> wrote:
I would have expected lvm to take that exclusive lock implicitely
when necessary?
I would not expect that. LVM provides means to activate in a shared or
exclusive way, but it does not choose itself.

I could live with that if it would be possible to elevate a LV from normal active to exclusive manually, but that seems not possible ATM. Actually I guess that lvm _does_ perform the exclusive lock implicitely, if it can (i.e. lv inactive on all nodes)


When the LV is exclusively activated (lvchange -aey) snapshots
should work (and they do work for me).
The volume is "lvchange -aly" active on one node and in use there
(e.g. mounted or attached to a VM). If I try to lvchange -aey on that
node, I get "Error locking on node xxxx: Device or resource busy".
I do not use '-aly' (in fact I am not sure what it does), so I cannot
relate.
It activates only on the local node; other nodes might enable as well if necessary.

Actually, lock exclusive will even fail if the device is not in use,
but only active locally.

A workaround would probably be to activate the lv exclusively
_before_ using it, but then it would be impossible to migrate the vm
to another host later on.
I use clustered LVM for my VMs too and always use "-aey" locking mode –
each volume can be active on a single cluster node a time. When doing VM
migration I first deactivate the volume on one host, then activate
it (exclusively) on the other. I feel safer knowing none of the volumes
will ever be active on more than one host.

Is your scenario much different?

You're obviously doing offline migration. While migrating live, there's a period with two nodes needing the volume being active. Thus exclusive locking would prevent live migration.

Regards
Andreas


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