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

[Anthony Liguori]

On 03/13/2011 09:28 PM, Chunqiang Tang wrote:
> > > In short, FVD's internal snapshot achieves the ideal properties of
> G1-G6,
> > > by 1) using the reference count table to only track "static"
> snapshots, 2)
> > > not keeping the reference count table in memory, 3) not updating the
> > > on-disk "static" reference count table when the VM runs, and 4)
> > > efficiently tracking dynamically allocated blocks by piggybacking on
> FVD's
> > > other features, i.e., its journal and small one-level lookup table.
> > Are you assuming snapshots are read-only?
> >
> > It's not clear to me how this would work with writeable snapshots.  It's
> > not clear to me that writeable snapshots are really that important, but
> > this is an advantage of having a refcount table.
> >
> > External snapshots are essentially read-only snapshots so I can
> > understand the argument for it.
> By definition, a snapshot itself must be immutable (read-only), but a
> writeable
> image state can be derived from an immutable snapshot by using
> copy-on-write,
> which I guess is what you meant by "writeable snapshot."

No, because the copy-on-write is another layer on top of the snapshot 
and AFAICT, they don't persist when moving between snapshots.

The equivalent for external snapshots would be:

base0 <- base1 <- base2 <- image

And then if I wanted to move to base1 without destroying base2 and 
image, I could do:

qemu-img create -f qcow2 -b base1 base1-overlay.img

The file system can keep a lot of these things around pretty easily but 
with your proposal, it seems like there can only be one.  If you support 
many of them, I think you'll degenerate to something as complex as a 
reference count table.

On the other hand, I think it's reasonable to just avoid the CoW overlay 
entirely and say that moving to a previous snapshot destroys any of it's 
children.  I think this ends up being a simplifying assumption that is 
worth investigating further.

From the use-cases that I'm aware of (backup and RAS), I think these 
semantics are okay.

I'm curious what other people think (Kevin/Stefan?).

Regards,

Anthony Liguori