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Date: Fri, 16 Nov 2012 09:22:39 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/7] raw-posix: add raw_get_aio_fd() for
 virtio-blk-data-plane
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Khoa Huynh <khoa@us.ibm.com>, Stefan Hajnoczi <stefanha@redhat.com>, Asias He <asias@redhat.com>

Il 16/11/2012 07:15, Stefan Hajnoczi ha scritto:
>> >
>> > 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.

Agreed.  This is not vhost-blk, for which I agree that opening the file
would make more sense (so you have no BDS at all).  It's just a stopgap
measure for something that should become the standard implementation.

Paolo