[sheepdog] [PATCH v2 06/11] sheep: decrement generational reference count on copy-on-write

[MORITA Kazutaka]

At Sun, 30 Jun 2013 14:20:18 +0800,
Kai Zhang wrote:
> 
> Based on my understanding, the new way of copy-on-write will be more costly than before.
> We will do two more gateway write operations for each object.

If the client send the old vdi id in the write request header, we can
decrease the number of additional write operations from two to one.  I
already implemented the optimization, but think of sending it after
this series is merged to make this series as simple as I can.

> 
> Did you do some performance testing? Do you think this will drop performance?
> (Although the performance drop would be covered by enabling cache, I still what
> to know how it will affect the performance when cache is disabled)

Not yet.  I'll add the performance number in the next version, but I'm
not sure how it is important since CoW is an expensive operation even
without this series.

Thanks,

Kazutaka