Re: [Xen-users] Xen 2.0 and LVM

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> my Linux Xen domU machine have its filesystems on LVM volumes provided
> by dom0. I wonder what's the overhead of doing this. In my understanding
> all disk access must go through dom0 because only that knows about the
> LVM layout. Is that correct?

Yes.  It's transparent to the guest how its data is being stored.

Block accesses will *always* have to go through dom0 unless you dedicate a 
disk controller to the guest and give it a separate set of physical disks.

> Now I'm lookig for a way to optimize the IO performance for domU:
> - Should I allocate more memory in dom0 for it's disk buffers? Are they
> involved at all?

You shouldn't have any performance probs.  LVM is an efficient backing storage 
for guest block devices, as are partitions and whole disks.  Using dom0 files 
as guest disk storage is inefficient and should be avoided on high-throughput 
systems.

> - Can I dedicate whole disks (hdb, hdc) to one domU and let it handle
> all disk access including LVM itself without involving dom0 at all?

You can give the guest a whole IDE controller and associated disks using the 
PCI virtualisation stuff if you really want to - I doubt it's worth it 
though.  You shouldn't be seeing much (any?) noticeable disk overhead for 
typical workloads.

HTH,
Mark

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