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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 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


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