Re: [Xen-users] AoE LVM2 DRBD Xen Setup

[Javier Guerra <javier@xxxxxxxxxxx>]

On Thursday 12 October 2006 7:25 am, Bjoern Metzdorf wrote:
> > You need CLVM ("Cluster LVM") from Redhat-Cluster-framework. No need for
> > GFS.
>
> Ok, what exactly is the clvm daemon for? Propagating changes to the VGs
> to all nodes accessing the VGs? Is it possible to do this manually with
> vgscan or such?

clvm isn't a daemon, it's just a couple of libraries and a few changes to 
lvm.conf:

       locking_library = "liblvm2clusterlock.so"
       locking_type = 2
       library_dir = "/lib/lvm2"

in the global{} block.  this libraries link with the cluster management tools 
of GFS (but doesn't need the whole package).

you can't just "do vgscan on all nodes", because between the time you do any 
change on the VG and the moment you do vgscan on the other nodes there MUST 
NOT be any access to the VG on any node.

the LVM docs say that it's safe to do:

        vgchange <vgname> -a n            on all but one node
        <do wanted changes on the only active node>
        vgchange <vgname> -a y            on the other nodes

but that would mean stopping all work on all nodes.

the CLVM libraries simply use the cluster tools to propagate a lock to suspend 
all nodes for a few miliseconds, apply the change and then release the lock.  
that way you can issue LVM administration commands from any node and it would 
be applied atomically on all nodes.

once it's installed it just works, well worth the trouble of setting up the 
cluster

-- 
Javier

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