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

[Andreas Pflug <pgadmin@pse-consulting.de>]

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