[ClusterLabs] how to "switch on" cLVM ?

[Lentes, Bernd]

----- On Jun 7, 2016, at 3:53 PM, Ferenc Wágner wferi at niif.hu wrote:

> "Lentes, Bernd" <bernd.lentes at helmholtz-muenchen.de> writes:
> 
>> Ok. Does DLM takes care that a LV just can be used on one host ?
> 
> No.  Even plain LVM uses locks to serialize access to its metadata
> (avoid concurrent writes corrupting it).  These locks are provided by
> the host kernel (locking_type=1).  DLM extends the locking concept to a
> full cluster from a single host, which is exactly what cLVM needs.  This
> is activated by locking_type=3.

So DLM and cLVM just takes care that the metadata is consistent.
None of them controls any access to the LV itself ?

> 
>> cLVM just takes care that the naming is the same on all nodes, right?
> 
> More than that.  As above, it keeps the LVM metadata consistent amongst
> the members of the cluster.  It can also activate LVs on all members
> ("global" activation), or ensure that an LV is active on a single member
> only ("exclusive" activation).
> 
>>>> 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?
> 
> The clvm daemon can be handled as a cloned cluster resource, but it
> isn't necessary.  It requires corosync (or some other membership/
> communication layer) and DLM to work.  DLM can be configured to do its
> own fencing or to use that of Pacemaker (if present).
> 
>>>> 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 ?
> 
> If your filesystem is a plain cluster resource, then your resource
> manager will ensure that it isn't mounted on more than one node, and
> everything should be all right.
> 
> Same with VMs on LVs: assuming no LV is used by two VMs (which would
> bring back the previous problem on another level) and your VMs are
> non-clone cluster resources, your resource manager will ensure that each
> LV is used by a single VM only (on whichever host), and everything
> should be all right, even though your LVs are active on all hosts (which
> makes live migration possible, if your resource agent supports that).
> --

Does the LV need to be a ressource (if i don't have a FS) ?
>From what i understand from what you say the LV's are active on all hosts,
and the ressource manager controls that a VM is just running on one host, so the LV
is just used by one host. Right ? So it has not to be a ressource.

Bernd

 

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