Re: [Qemu-devel] [RFC PATCH v2 00/23] qcow2: Delayed COW

[Stefan Hajnoczi]

On Wed, Feb 13, 2013 at 02:21:50PM +0100, Kevin Wolf wrote:
> During the last few releases we have got rid of most of the overhead of
> metadata writes during cluster allocation. What's left is the COW for
> unaligned allocating write requests, and it's quite expensive.
> 
> In the general case, this cost cannot be avoided. However, if we're
> lucky enough that before the next flush the data copied during COW
> would be overwritten, we can do without the COW. Sequential writes
> always overwrite the COW area at the end of the cluster immediately,
> so delaying the COW a bit and cancelling it if it's overwritten is a
> worthwhile optimisation.

This series increases the complexity of qcow2 request processing by
adding locks, state variables, and expanding the life-cycle of requests.

My reaction to this RFC is that I'm a little scared not just of possible
bugs I'll miss during code review, but also that future qcow2
modifications will get even trickier due to the new state.

I hope to come up with some concrete ideas for simplifying this as I
iterate over the patches some more.

Stefan