Re: [Xen-users] Linux software RAID1 (Dom0 or domU)?

["Luke S. Crawford" <lsc@xxxxxxxxx>]

On Thu, 1 Feb 2007, Mark Williamson wrote:
> I think I'd do RAID in dom0...  It makes configuration changes easy to do
> without fiddling with the guest and will free up some space in the block ring
> (although I'm not convinced that'll make much difference to performance).

Hm.  I'm interested in this discussion because I have a fibre-channel 
setup with lots of disks, but no raid head.  Right now, we are running md 
software raids in Dom0 as you suggest;  the primary problem with this is 
that hot-migration won't work; all my Dom0s can see all the disks, (well, 
depending on how I setup zoning)   but the MD devices only exist on the 
server that created them, so I believe hot-migration or even having half 
of one md on one server and half of the same md on another server just 
won't work.
One way to solve this is to make clvm support mirroring;  but I'm 
too dumb to write that code myself.
the other solution would be to make every physical disk a clvm 
VolumeGroup;  then make sure that every DomU have 2 equaly-sized 
partitions from different volume groups.  The DomU can then mirror or 
stripe the disks as required.
(this has actually been a pretty hot internal prgmr.com debate;  the best 
solution, we all agree, is to get a hardware-raid device, but we haven't 
done so yet, mostly because I'm cheap.  disks and fibre switches are 
commodity, raid-heads are decidedly not.)
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