Am 30.06.2011 13:48, schrieb Stefan Hajnoczi: > On Wed, Jun 29, 2011 at 4:41 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >> On Wed, Jun 29, 2011 at 11:08:23AM +0100, Stefan Hajnoczi wrote: >>> In the future we could add a 'base' argument to block_stream. If base >>> is specified then data contained in the base image will not be copied. >> >> This is a present requirement. > > It's not one that I have had in the past but it is a reasonable requirement. > > One interesting thing about this requirement is that it makes > copy-on-read seem like the wrong primitive for image streaming. If > there is a base image which should not be streamed then a plain loop > that calls bdrv_is_allocated_chain(bs, base, sector, &pnum) and copies > sectors into bs is more straightforward than passing base to a > copy-on-read operation somehow (through a variable that stashes the > base away somewhere?). You don't even have to look at the implementation to say that COR is a useful optimisation. It basically means that you reuse data read by the guest instead of reading it a second time in your loop. (And this is equally true for block copy and image streaming) If this means adding a new field in BlockDriverState, so be it. Kevin -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
