Zoned storage support in libvirt

[Daniel P. Berrangé]

On Tue, Jan 10, 2023 at 10:19:51AM -0500, Stefan Hajnoczi wrote:
> Hi Peter,
> Zoned storage support
> (https://zonedstorage.io/docs/introduction/zoned-storage) is being added
> to QEMU. Given a zoned host block device, the QEMU syntax will look like
> this:
> 
>   --blockdev zoned_host_device,node-name=drive0,filename=/dev/$BDEV,...
>   --device virtio-blk-pci,drive=drive0
> 
> Note that regular --blockdev host_device will not work.
> 
> For now the virtio-blk device is the only one that supports zoned
> blockdevs.

Does the virtio-blk device expowsed guest ABI differ at all
when connected zoned_host_device instead of host_device ?

> This brings to mind a few questions:
> 
> 1. Does libvirt need domain XML syntax for zoned storage? Alternatively,
>    it could probe /sys/block/$BDEV/queue/zoned and generate the correct
>    QEMU command-line arguments for zoned devices when the contents of
>    the file are not "none".
> 
> 2. Should QEMU --blockdev host_device detected zoned devices so that
>    --blockdev zoned_host_device is not necessary? That way libvirt would
>    automatically support zoned storage without any domain XML syntax or
>    libvirt code changes.
> 
>    The drawbacks I see when QEMU detects zoned storage automatically:
>    - You can't easiy tell if a blockdev is zoned from the command-line.
>    - It's possible to mismatch zoned and non-zoned devices across live
>      migration.

What happens with existing QEMU impls if you use --blockdev host_device
pointing to a /dev/$BDEV that is a zoned device ?  If it succeeds and
works correctly, then we likely need to continue to support that. This
would push towards needing a new XML element.

With regards,
Daniel
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