[vox-tech] Xen + LVM usage

[Luke Crawford]

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 
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.

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

--

It is a cool idea, though, esp. for backups.  I can xm pause; lvcreate; xm 
unpause;  to grab a quiecent copy for backups.

I've also been looking at GFS from the same people that wrote LFS... it 
would mean all my Dom0s would need to be linux, but then all my fibre 
disks would be seen as one massive disk that I could carve out named 
partitions from, named as the domUs. I could run xm migrate with impunity. 
I could also run my fibre cards in arbitrated-loop mode (cheaper cards and 
cheaper switches/hubs are available for that) rather than the fabric mode 
soft zones require.