[Xen-devel] of cows and clones: creating domains as clones of saved state

[Peri Hankey <mpah@xxxxxxxxxxxxxx>]

Xen can already save the state of an established domain and restore it 
(in theory at least - so far it crashes for me).  It seems to me that 
most of the objectives discussed for a copy-on-write memory system can 
be achieved by providing a mechanism for  creating domains by cloning 
them from an initial state that is shared read-only between them with a 
copy-on-write mapping for each.
In this scheme a collection of clone domains can be created by starting 
from the saved state X of some original domain, which may be configured 
specifically for this purpose. The saved state X encapsulates the memory 
and filesystem state of the originating domain. The memory and 
filesystem components of the state X are shared read-only between the 
clone domains of X, with each clone domain superimposing its own 
copy-on-write mapping of the memory and filesystem states
When a clone domain is started, it immediately reconfigures itself as a 
distinct independent domain by writing its own configuration data to the 
copy-on-write mapping of its initial memory and filesystem state.
Once it has reconfigured itself, the state of a clone domain is the 
state represented by the the copy-on-write mapping of its private data 
as an overlay on the shared state from which it was first created.
This scheme has the added merit that creating a new domain of a 
particular kind is simply a matter of creating a new clone domain and 
reconfiguring it as an independent domain. Creating a new domain should 
take not much longer than restarting a migrating domain in a different 
machine.
Once it has reconfigured itself as an independent domain, each clone 
domain operates in the same way as any other domain. In particular, each 
has a fixed allocation of memory pages, but these are available to it 
over and above the pages that it shares read-write with other clones of 
the same initial state.
As far as I can see, the only mechanism that is required from xen is a 
mechanism for sharing memory pages read-only between domains with a 
copy-on-write mapping for each domain. It may be that the copy-on-write 
part of this mechanism is best handled by each guest operating  system, 
although it would obviously be best to provide that as a service of the 
xen system itself.
There is an implied requirement that a clone domain created from a state 
X can only migrate to a machine where the the shared state X is available.
Regards
Peri Hankey






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