Re: [PATCH v5 1/4] docs/qcow2: add the zoned format feature

[Eric Blake]

On Mon, Oct 30, 2023 at 08:18:44PM +0800, Sam Li wrote:
> Add the specs for the zoned format feature of the qcow2 driver.
> The qcow2 file can be taken as zoned device and passed through by
> virtio-blk device or NVMe ZNS device to the guest given zoned
> information.
> 
> Signed-off-by: Sam Li <faithilikerun@gmail.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/system/qemu-block-drivers.rst.inc | 33 ++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/docs/system/qemu-block-drivers.rst.inc 
> b/docs/system/qemu-block-drivers.rst.inc
> index 105cb9679c..4647c5fa29 100644
> --- a/docs/system/qemu-block-drivers.rst.inc
> +++ b/docs/system/qemu-block-drivers.rst.inc
> @@ -172,6 +172,39 @@ This section describes each format and the options that 
> are supported for it.
>      filename`` to check if the NOCOW flag is set or not (Capital 'C' is
>      NOCOW flag).
>  
> +  .. option:: zoned
> +    1 for host-managed zoned device and 0 for a non-zoned device.

Should this be a bool or enum type, instead of requiring the user to
know magic numbers?  Is there a potential to add yet another type in
the future?

> +
> +  .. option:: zone_size
> +
> +    The size of a zone in bytes. The device is divided into zones of this
> +    size with the exception of the last zone, which may be smaller.
> +
> +  .. option:: zone_capacity
> +
> +    The initial capacity value, in bytes, for all zones. The capacity must
> +    be less than or equal to zone size. If the last zone is smaller, then
> +    its capacity is capped.
> +
> +    The zone capacity is per zone and may be different between zones in real
> +    devices. For simplicity, QCow2 sets all zones to the same capacity.

Just making sure I understand: One possible setup would be to describe
a block device with zones of size 1024M but with capacity 1000M (that
is, the zone reserves 24M capacity for other purposes)?

Otherwise, I'm having a hard time seeing when you would ever set a
capacity different from size.

Are there requirements that one (or both) of these values must be
powers of 2?  Or is the requirement merely that they must be a
multiple of 512 bytes (because sub-sector operations are not
permitted)?  Is there any implicit requirement based on qcow2
implementation that a zone size/capacity must be a multiple of cluster
size (other than possibly for the last zone)?

> +
> +  .. option:: zone_nr_conv
> +
> +    The number of conventional zones of the zoned device.
> +
> +  .. option:: max_open_zones
> +
> +    The maximal allowed open zones.
> +
> +  .. option:: max_active_zones
> +
> +    The limit of the zones with implicit open, explicit open or closed state.
> +
> +  .. option:: max_append_sectors
> +
> +    The maximal number of 512-byte sectors in a zone append request.

Why is this value in sectors instead of bytes?  I understand that
drivers may be written with sectors in mind, but any time we mix units
in the public interface, it gets awkward.  I'd lean towards having
bytes here, with a requirement that it be a multiple of 512.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org