Re: [Qemu-devel] Re: Strategic decision: COW format

[Chunqiang Tang]

> > Your use of "current-state" is confusing me because AFAICT, 
> > current-state is just semantically another snapshot.
> > 
> > It's writable because it has no children.  You only keep around one 
> > writable snapshot and to make another snapshot writable, you have to 
> > discard the former.
> > 
> > This is not the semantics of qcow2.  Every time you create a snapshot, 

> > it's essentially a new image.  You can write directly to it.
> > 
> > While we don't do this today and I don't think we ever should, it's 
> > entirely possible to have two disks served simultaneously out of the 
> > same qcow2 file using snapshots.
> 
> No, CQ is describing the semantics of internal snapshots in qcow2
> correctly. You have all the snapshots that are stored in the snapshot
> table (all read-only) plus one current state described by the image
> header (read-write).

That's also the semantics of VMware's external snapshot. So there is no 
difference in semantics. It is just a difference in implementation and 
performance.

Regards,
ChunQiang (CQ) Tang
Homepage: http://www.research.ibm.com/people/c/ctang