Re: [Xen-users] COW-backed VBDs

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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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