[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Zoned storage support in libvirt
From: |
Stefan Hajnoczi |
Subject: |
Re: Zoned storage support in libvirt |
Date: |
Wed, 11 Jan 2023 10:24:30 -0500 |
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
signature.asc
Description: PGP signature