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

[Stefan Hajnoczi]

On Tue, Jan 17, 2012 at 3:47 PM, Marcelo Tosatti <address@hidden> wrote:
> 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 <address@hidden>
>> >>>
>> >>> Add support for streaming data from an intermediate section of the
>> >>> image chain (see patch and documentation for details).
>> >>>
>> >>> Signed-off-by: Marcelo Tosatti <address@hidden>
>> >>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>> >>
>> >> 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.

Sounds like a good suggestion.  It will prevent the case where a guest
is doing heavy read I/O during image streaming with a 'base' and we
bloat the destination image file.

I'll resend the series with this fix.

Stefan