On Tue, Jan 10, 2023 at 03:29:47PM +0000, Daniel P. Berrangé wrote:
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/$BDEV,... --device virtio-blk-pci,drive=drive0
Note that regular --blockdev host_device will not work.
For now the virtio-blk device is the only one that supports zoned blockdevs.
Does the virtio-blk device expowsed guest ABI differ at all when connected zoned_host_device instead of host_device ?
Yes. There is a VIRTIO feature bit, some configuration space fields, etc. virtio-blk-pci detects when the blockdev is zoned and enables the feature bit.
This brings to mind a few questions:
1. Does libvirt need domain XML syntax for zoned storage? Alternatively, it could probe /sys/block/$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.
Pointing host_device at a zoned device doesn't result in useful behavior because the guest is unaware that this is a zoned device. The guest won't be able to access the device correctly (i.e. sequential writes only). Write requests will fail eventually. I would consider zoned devices totally unsupported in QEMU today and we don't need to worry about preserving any kind of backwards compatibility with --blockdev host_device,filename=/dev/my_zoned_device. Stefan