Re: Use LVM on /dev/mapper/diskname and iSCSI

[Tore Anderson <tore@linpro.no>]

* S. J. van Harmelen

> Thanks for your help!
> 
> I executed the pvs command and it did find a few partitions. I use
> iSCSI to share the LUN's to a XenSource server, and XenSource creates
> the PV's and LV's in order to provision VM's. Didn't think of that,
> but that's on the disk of course.
> 
> Is there any way to configure snapshots for these already created LVM
> partitions? Or do I have to whipe the disk clean (rather not do
> that, but if I have to it can be done), then create the pv's and lv's
> and then share the lv's true iSCSI?
> 
> In that case XenSource will again create pv's and lv's in the shared
> lv. Ain't that a problem?

You really need a filter line in /etc/lvm/lvm.conf, to keep LVM from 
claiming 1) individual paths to a dm-multipath controlled LUN, and 2) 
LUNs that are PVs used by the LVM implementation in your domUs.

You don't want LVM to be claiming the paths directly, because you'd no 
longer have any redundancy - it will override dm-multipath.  You also 
don't want the LVM implementation in your dom0 to be using the same 
block device as LVM in your domU as physical volumes, as LVM is designed 
to have exclusive access to its PVs (you might get around this with 
CLVM, though).

Do I understand correctly if you have one LUN that contains your domO 
(having its file systems in LVs), and several LUNs that contain your 
domUs, which again are PVs that are supposed to be used by your domU 
kernels?

If so you will want to keep your dom0 from claiming the domU PVs.  Use a 
filter line that accepts only the multipath'ed volume that contains the 
dom0 installation, and drop the rest.  For instance:

filter = [ "a|^/dev/mapper/domzero$", "r|.*" ]

...assuming your dom0 data volume is called "domzero" in multipath.conf.

Regards
-- 
Tore Anderson