[ClusterLabs] how to "switch on" cLVM ?

[Lentes, Bernd]

----- On Jun 6, 2016, at 8:17 PM, Digimer lists at alteeve.ca wrote:

> On 06/06/16 01:13 PM, Lentes, Bernd wrote:
>> Hi,
>> 
>> i'm currently establishing a two node cluster. I have a FC SAN and two hosts. My
>> services are integrated into virtual machines (kvm). The vm's should reside on
>> the SAN.
>> The hosts are connected to the SAN via FC HBA. Inside the hosts i see already
>> the volume from the SAN. I'd like to store each vm in a dedicated logical
>> volume (with or without filesystem in the LV).
>> Hosts are SLES 11 SP4. LVM2 and LVM2-clvm is installed.
>> How do i have to "switch on" clvm ? Is locking_type=3 in /etc/lvm/lvm.conf all
>> which is necessary ?
> 
> That tells LVM to use cluster locking, you still need to actually
> provide the cluster locking, called DLM. With the cluster formed (and
> fencing working!), you should be able to start the clvmd daemon.
> 
>> In both nodes ?
> 
> Yes
> 
>> Restart of the init-scripts afterwards sufficient ?
> 
> No, DLM needs to be added to the cluster and be running.

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 ?

> 
>> And how do i have to proceed afterwards ?
>> My idea is:
>> 1. Create a PV
>> 2. Create a VG
>> 3. Create several LV's
> 
> If the VG is created while dlm is running, it should automatically flag
> the VG as clustered. If not, you will need to tell LVM that the VG is
> clustered (-cy, iirc).
> 
>> And because of clvm i only have to do that on one host and the other host sees
>> all automatically ?
> 
> Once clvmd is running, any changes made (lvcreate, delete, resize, etc)
> will immediately appear on the other nodes.
> 

Ok.

>> 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 ?

Bernd
 

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