Re: [libvirt] [PATCH 1/6] Add new API virDomainStreamDisk[Info] to header and drivers
On 04/08/2011 11:02 AM, Stefan Hajnoczi wrote:
On Fri, Apr 8, 2011 at 2:31 PM, Daniel P.
Berrange<berrange(a)redhat.com> wrote:
I have CCed Anthony and Kevin. Anthony drove the QED image streaming
and Kevin will probably be interested in the idea of allocating raw
images as a background activity while QEMU runs.
> /*
> * @path: fully qualified filename of the virtual disk
> * @nregions: filled in the number of @region structs
> * @regions: filled with a list of allocated regions
> *
> * Query the extents of allocated regions within the
> * virtual disk file. The offsets in the list of regions
> * are not guarenteed to be sorted in any explicit order.
> */
> int virDomainBlockGetAllocationMap(virDomainPtr dom,
> const char *path,
> unsigned int *nregions,
> virDomainBlockRegionPtr *regions);
QEMU can provide this with its existing .bdrv_is_allocated() function.
Kevin, do you have any thoughts on whether this API will work well?
I think the trouble with this API proposal is that it's overloading
concepts.
Sparse is not the same thing as CoW to a backing file.
For instance, when you expose streaming, the result is still a sparse
file. So you'd have a rather curious API where you called to "allocate"
a region in the file which resulted in having a sparse file which you
then called again to make it non sparse. But AFAICT, the API doesn't
really tell you these details.
I think it's too much abstraction with not enough commonality.
Having to related APIs to expand a copy-on-read image and then to fill
in a sparse file is certainly a reasonable thing to do. I think trying
to make a single API that does both without having a flag that basically
makes it two APIs is going to be cumbersome.
Regards,
Anthony Liguori