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
Attachment:
signature.asc
Description: PGP signature
