RE: [Xen-devel] Re: Questions on Copy-on-Write

["Williamson, Mark A" <mark.a.williamson@xxxxxxxxx>]

> Doesn't this sit at the wrong level?  This would make all 
> disk accesses on
> these devices go through Domain0?

That's not quite right: virtual block devices are an abstraction
provided by Xen and domains that use them talk to Xen directly, without
going through Dom0.  VBDs are a way of securely sharing portions of the
physical disk space in the system between domains - Dom0 creates VBDs to
tell Xen what domains can access what parts of the disk.  Those domains
then talk to Xen directly to access those areas.

A domain could be given read only access to the VBD with a "standard
image" for all domains on it and read-write access to another VBD for
it's own personal use.  Using Bin's CoW driver, the domain can then
store all its updates to the standard image in the read-write VBD.  The
domain then talks directly to Xen in order to access these VBDs, same as
usual.

HTH,

Mark


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