[ClusterLabs] how to "switch on" cLVM ?

Lentes, Bernd bernd.lentes at helmholtz-muenchen.de
Thu Jun 9 09:29:51 EDT 2016

----- On Jun 7, 2016, at 11:52 PM, Ferenc Wágner wferi at niif.hu wrote:

> "Lentes, Bernd" <bernd.lentes at helmholtz-muenchen.de> writes:
>> ----- 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 contols activation as well (besides metadata consistency), but does
> not control access to activated LVs, which are cluster-unaware
> device-mapper devices, just like under plain LVM.
>>>> 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) ?
> No.  (If you use cLVM.  If you don't use cLVM, then your VGs must be
> resources, otherwise nothing guarrantees the consistency of their
> metadata.)
>> 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.
> Right.  (The LVs must be active on all hosts to enable free live
> migration.  There might be other solutions, because the LVs receive I/O
> on one host only at any given time, but then you have to persuade your
> hypervisor that the block device it wants will really be available once
> migration is complete.)
> --
> Feri

OK. I think i got it now.
Thanks for the help.


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