> I have asked this question before about COW backed VBD support in Xen. It
> would be really cool if I could share the same root FS between domains,
> each having its own COW system.
Indeed.
> I am typically using file-backed roots. Does Xen (as User-mode linux)
> provide utility to create COW disks (linked to a readOnly main disk) which
> can be used as VBDs.
You can use LVM2 snapshots or the Device Mapper to create a COW virtual 
device. LVM, at least, seems to have some issues with scalability when doing 
lots of CoW-ing though - make sure dom0 has enough memory.
The other way of doing things is CoW at the filesystem, rather than at block 
level. E.g. combine NFS and UnionFS or use a CoW NFS server (someone in 
Cambridge developed one but I don't know if the code is floating around). 
I'll be working on a CoW filesystem also, later this year.
Cheers,
Mark
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