[RFC PATCH 0/2] virtio nvme

[Stefan Hajnoczi]

On Fri, Sep 11, 2015 at 6:21 PM, Ming Lin <mlin at kernel.org> wrote:
> On Fri, 2015-09-11 at 08:48 +0100, Stefan Hajnoczi wrote:
>> On Thu, Sep 10, 2015 at 6:28 PM, Ming Lin <mlin at kernel.org> wrote:
>> > On Thu, 2015-09-10 at 15:38 +0100, Stefan Hajnoczi wrote:
>> >> On Thu, Sep 10, 2015 at 6:48 AM, Ming Lin <mlin at kernel.org> wrote:
>> >> > These 2 patches added virtio-nvme to kernel and qemu,
>> >> > basically modified from virtio-blk and nvme code.
>> >> >
>> >> > As title said, request for your comments.
>> >> >
>> >> > Play it in Qemu with:
>> >> > -drive file=disk.img,format=raw,if=none,id=D22 \
>> >> > -device virtio-nvme-pci,drive=D22,serial=1234,num_queues=4
>> >> >
>> >> > The goal is to have a full NVMe stack from VM guest(virtio-nvme)
>> >> > to host(vhost_nvme) to LIO NVMe-over-fabrics target.
>> >>
>> >> Why is a virtio-nvme guest device needed?  I guess there must either
>> >> be NVMe-only features that you want to pass through, or you think the
>> >> performance will be significantly better than virtio-blk/virtio-scsi?
>> >
>> > It simply passes through NVMe commands.
>>
>> I understand that.  My question is why the guest needs to send NVMe commands?
>>
>> If the virtio_nvme.ko guest driver only sends read/write/flush then
>> there's no advantage over virtio-blk.
>>
>> There must be something you are trying to achieve which is not
>> possible with virtio-blk or virtio-scsi.  What is that?
>
> I actually learned from your virtio-scsi work.
> http://www.linux-kvm.org/images/f/f5/2011-forum-virtio-scsi.pdf
>
> Then I thought a full NVMe stack from guest to host to target seems
> reasonable.
>
> Trying to achieve similar things as virtio-scsi, but all NVMe protocol.
>
> - Effective NVMe passthrough
> - Multiple target choices: QEMU, LIO-NVMe(vhost_nvme)
> - Almost unlimited scalability. Thousands of namespaces per PCI device
> - True NVMe device
> - End-to-end Protection Information
> - ....

The advantages you mentioned are already available in virtio-scsi,
except for the NVMe command set.

I don't understand what unique problem virtio-nvme solves yet.  If
someone asked me to explain why NVMe-over-virtio makes sense compared
to the existing virtio-blk/virtio-scsi or NVMe SR-IOV options, I
wouldn't know the answer.  I'd like to learn that from you or anyone
else on CC.

Do you have a use case in mind?

Stefan