[ClusterLabs] how to "switch on" cLVM ?

Lentes, Bernd bernd.lentes at helmholtz-muenchen.de
Tue Jun 7 07:23:03 EDT 2016

----- On Jun 7, 2016, at 12:28 PM, Sven Moeller smoeller at nichthelfer.de wrote:

> Hi,
> Am Dienstag, 07. Juni 2016 11:42 CEST, "Lentes, Bernd"
> <bernd.lentes at helmholtz-muenchen.de> schrieb:
>> ----- On Jun 6, 2016, at 8:17 PM, Digimer lists at alteeve.ca wrote:

>> Ok. Does DLM takes care that a LV just can be used on one host ?
>> cLVM just takes care that the naming is the same on all nodes, right ?
> AFAIK DLM takes care about the LVM Locking cluster wide.

What does that mean concretely, "LVM Locking cluster wide" ?
I read it always, but no explaination.
Does that mean that a LV just can be accesses from one node ?
That DLM "locks" (in the sense of "blocking the other node") the access ?
Or does the LV has to be a cluster ressource to prevent concurrent access ?

>> >> Later on it's possible that some vm's run on host 1 and some on host 2. Does
>> >> clvm needs to be a ressource managed by the cluster manager ?
>> > 
>> > Yes, you can live-migrate as well. I do this all the time, except I use
>> > DRBD instead of a SAN and RHEL instead of SUSE, but those are trivial
>> > differences in this case.
>> > 
>> >> If i use a fs inside the lv, a "normal" fs like ext3 is sufficient, i think. But
>> >> it has to be a cluster ressource, right ?
>> > 
>> > You can format a clustered LV with a cluster unaware filesystem just
>> > fine. However, the FS is not made magically cluster aware... If you
>> > mount it on two nodes, you will almost certainly corrupt the FS quickly.
>> > If you want to mount an LV on two+ nodes at once, you need a
>> > cluster-aware file system, life GFS2.
>> No. Pacemaker takes care that the FS is just mounted on one node.
>> So it should not be a problem ?
> If you want to be sure to mount an LV on just one Node, you have to activate the
> VG exclusively on one node. You have to configure the ressource for the VG
> accordingly. Otherwise it's possible to activate and mount an LV on several
> nodes at the same time, even with a non Cluster FS, e.g. ext4, which would end
> up in corrupted FS, most likely. (as mentioned above allready).

But maybe i want to have some vm's running on host A and others on host B.
Remember: one vm per LV.
So i need access to the VG concurrently from both nodes, right ?
But if the FS from the LV is a cluster ressource, pacemaker takes care the the FS is mounted just from one node.
I can rely it on it, right ? That's what i read often.
But what if i don't have a FS ? It's possible to have vm's in plain partitions, which should be a performance advantage.



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