Re: [Qemu-devel] [PATCH 1/7] raw-posix: add raw_get_aio_fd() for virtio-blk-data-plane

[Stefan Hajnoczi]

On Thu, Nov 15, 2012 at 9:03 PM, Anthony Liguori <address@hidden> wrote:
> Stefan Hajnoczi <address@hidden> writes:
>
>> The raw_get_aio_fd() function allows virtio-blk-data-plane to get the
>> file descriptor of a raw image file with Linux AIO enabled.  This
>> interface is really a layering violation that can be resolved once the
>> block layer is able to run outside the global mutex - at that point
>> virtio-blk-data-plane will switch from custom Linux AIO code to using
>> the block layer.
>>
>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>
> I think this creates user confusion because virtio-blk-data-plane can't
> actually take a BDS.
>
> So why not just make a string 'filename' property and open it directly
> in virtio-blk-data-plane?  Then it's at least clear to the user and
> management tools what the device is capable of doing.

There are some benefits to raw_get_aio_fd():

1. virtio-blk-data-plane is only a subset virtio-blk implementation,
it still needs a regular virtio-blk-pci device (with BDS) in order to
run.  If we use a filename the user would have to specify it twice.

2. Fetching the file descriptor in this way ensures that the image
file is format=raw.

3. virtio-blk-data-plane uses Linux AIO and raw-posix.c has checks
which I don't want to duplicate - we can simply check s->use_aio in
raw_get_aio_fd() to confirm that Linux AIO can be used.

If we open a file directly then we lose these benefits.  Do you still
think we should open a filename?

Stefan