Re: [Xen-users] Re: LVM on DRBD -OR- DRBD on LVM?

[Javier Guerra <javier@xxxxxxxxxxx>]

On Wednesday 21 June 2006 5:12 pm, xen-users@xxxxxxxxxxxxx wrote:
> besides, unless you are running something like RedHat's Cluster LVM
> (clvm), the plain old LVM doesn't understand multi access
> (active/active) VGs yet, I don't think.

not exactly true;  you can certainly use plain LVM on several nodes using the 
same PV, just as long as:

a) no two nodes use the same LV at the same time

and,

b) can't modify the LVM metadata while other nodes use it.

IOW:
1) use pvcreate, vgcreate, lvcreate, lvextend, etc on _one_ node.
2) do a vgscan on all nodes; all see the LVs, use them as desired; just don't 
fight over a single LV
3) to do ANY modification to the VG, deactivate all nodes except one (shut 
down or "vgchange -a n")
4) use lvcreate, lvresize, etc. on the only node left
5) do "vgscan; vgchange -a y" on the other nodes (or turn them on)

the point is that the only really 'shared' data is the LVM metadata; so use 
it 'read only'.

the clvm package (it's on Debian and Ubuntu, besides RH and Centos) uses the 
GFS cluster locking to arbitrate access to the LVM metadata.  with it, you 
can use LVM commands on any node, and all would see the changes immediately 
and without conflicts (except pvmove, i think).  each LV contents goes as 
usual, that is, don't mount at more than one node (unless you put a cluster 
filesystem on it).

-- 
Javier

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