[vox-tech] Xen + LVM usage

[Bill Broadley]

On Tue, Aug 08, 2006 at 05:30:04PM -0700, Luke Crawford wrote:
> On Mon, 7 Aug 2006, Bill Broadley wrote:
> >Ah.  Well the Xen manual recommends:
> > To create two copy-on-write clones of the above file system you would use 
> > the   following commands:
> >
> >  # lvcreate -s -L1024M -n myclonedisk1 /dev/vg/myvmdisk1
> >  # lvcreate -s -L1024M -n myclonedisk2 /dev/vg/myvmdisk1
> >
> >So they are actually parititions, not layered on top of a filesystem.  I
> >figure the Xen folks wouldn't recommend it if it didn't work well.
> 
> I wouldn't go so far as to say they recommend it... here is some context:
> 
> You can also use LVM for creating copy-on-write (CoW) clones of LVM 
> volumes (known as writable persistent snapshots in LVM terminology). This 
> facility is new in Linux 2.6.8, so isn't as stable as one might hope. In 

New in August 2004 isn't exactly bleeding edge.

> particular, using lots of CoW LVM disks consumes a lot of dom0 memory, and 
> error conditions such as running out of disk space are not handled well. 
> Hopefully this will improve in future.

I wondered what they meant by that.  Seems like running out of disk is
always a disaster.  I tried it a few times and I couldn't write anymore,
the biggest issue I had is you can't tell how much you have left without
looking.  Seems rather user unfriendly to have df be unrelated to
remaining disk space.

I've played with it some, but disk is so cheap I've defaulted to
just "wasting" a few GB for the OS install and then a LVM volume
for the data (spool for a mailserver, homedirs for fileservers,
packages for app servers, etc.).

-- 
Bill Broadley
Computational Science and Engineering
UC Davis