Re: [Qemu-devel] [PATCH v5 13/15] block: add support for partial streaming

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jan 17, 2012 at 03:05:29PM +0100, Kevin Wolf wrote:
> Am 17.01.2012 14:50, schrieb Marcelo Tosatti:
> > On Tue, Jan 17, 2012 at 02:27:04PM +0100, Kevin Wolf wrote:
> >> Am 13.01.2012 14:14, schrieb Stefan Hajnoczi:
> >>> From: Marcelo Tosatti <mtosatti@redhat.com>
> >>>
> >>> Add support for streaming data from an intermediate section of the
> >>> image chain (see patch and documentation for details).
> >>>
> >>> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >>> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> >>
> >> I'm afraid that in the review for the previous version I couldn't see
> >> the wood for the trees... This does limit the COR requests issued by
> >> image streaming, but not those issued by the guest. Am I missing
> >> something? This is not what we want, is it?
> > 
> > What you mean "limit the COR requests"? 
> 
> base -> sn1 -> sn2
> 
> You only want to copy the content of sn1 into sn2 and keep base. The
> streaming coroutine is doing the right thing because it checks
> is_allocated_base. However, if it is the guest that reads some data from
> base, COR copies it into sn2 even though it's in the common base file.

Ah, yes.

> Maybe streaming shouldn't enable normal COR on images, but instead of
> calling bdrv_co_read it could directly call bdrv_co_copy_on_readv().

That would work.