Re: [PATCH v15 8/8] docs/zoned-storage: add zoned device documentation

[Stefan Hajnoczi]

On Sun, 29 Jan 2023 at 05:31, Sam Li <faithilikerun@gmail.com> wrote:
>
> Add the documentation about the zoned device support to virtio-blk
> emulation.
>
> Signed-off-by: Sam Li <faithilikerun@gmail.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
> Reviewed-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
> ---
>  docs/devel/zoned-storage.rst           | 43 ++++++++++++++++++++++++++
>  docs/system/qemu-block-drivers.rst.inc |  6 ++++
>  2 files changed, 49 insertions(+)
>  create mode 100644 docs/devel/zoned-storage.rst

This patch uses the old "zoned_host_device" BlockDriver name. Please
update it to "host_device".

It's probably a good idea to search the patches for zoned_host_device
to find any other places that need to be updated. That can be done
with git log -p master.. | grep zoned_host_device.

> diff --git a/docs/devel/zoned-storage.rst b/docs/devel/zoned-storage.rst
> new file mode 100644
> index 0000000000..03e52efe2e
> --- /dev/null
> +++ b/docs/devel/zoned-storage.rst
> @@ -0,0 +1,43 @@
> +=============
> +zoned-storage
> +=============
> +
> +Zoned Block Devices (ZBDs) divide the LBA space into block regions called 
> zones
> +that are larger than the LBA size. They can only allow sequential writes, 
> which
> +can reduce write amplification in SSDs, and potentially lead to higher
> +throughput and increased capacity. More details about ZBDs can be found at:
> +
> +https://zonedstorage.io/docs/introduction/zoned-storage
> +
> +1. Block layer APIs for zoned storage
> +-------------------------------------
> +QEMU block layer supports three zoned storage models:
> +- BLK_Z_HM: The host-managed zoned model only allows sequential writes access
> +to zones. It supports ZBD-specific I/O commands that can be used by a host to
> +manage the zones of a device.
> +- BLK_Z_HA: The host-aware zoned model allows random write operations in
> +zones, making it backward compatible with regular block devices.
> +- BLK_Z_NONE: The non-zoned model has no zones support. It includes both
> +regular and drive-managed ZBD devices. ZBD-specific I/O commands are not
> +supported.
> +
> +The block device information resides inside BlockDriverState. QEMU uses
> +BlockLimits struct(BlockDriverState::bl) that is continuously accessed by the
> +block layer while processing I/O requests. A BlockBackend has a root pointer 
> to
> +a BlockDriverState graph(for example, raw format on top of file-posix). The
> +zoned storage information can be propagated from the leaf BlockDriverState 
> all
> +the way up to the BlockBackend. If the zoned storage model in file-posix is
> +set to BLK_Z_HM, then block drivers will declare support for zoned host 
> device.
> +
> +The block layer APIs support commands needed for zoned storage devices,
> +including report zones, four zone operations, and zone append.
> +
> +2. Emulating zoned storage controllers
> +--------------------------------------
> +When the BlockBackend's BlockLimits model reports a zoned storage device, 
> users
> +like the virtio-blk emulation or the qemu-io-cmds.c utility can use block 
> layer
> +APIs for zoned storage emulation or testing.
> +
> +For example, to test zone_report on a null_blk device using qemu-io is:
> +$ path/to/qemu-io --image-opts -n 
> driver=zoned_host_device,filename=/dev/nullb0
> +-c "zrp offset nr_zones"
> diff --git a/docs/system/qemu-block-drivers.rst.inc 
> b/docs/system/qemu-block-drivers.rst.inc
> index dfe5d2293d..0b97227fd9 100644
> --- a/docs/system/qemu-block-drivers.rst.inc
> +++ b/docs/system/qemu-block-drivers.rst.inc
> @@ -430,6 +430,12 @@ Hard disks
>    you may corrupt your host data (use the ``-snapshot`` command
>    line option or modify the device permissions accordingly).
>
> +Zoned block devices
> +  Zoned block devices can be passed through to the guest if the emulated 
> storage
> +  controller supports zoned storage. Use ``--blockdev zoned_host_device,
> +  node-name=drive0,filename=/dev/nullb0`` to pass through ``/dev/nullb0``
> +  as ``drive0``.
> +
>  Windows
>  ^^^^^^^
>
> --
> 2.38.1
>
>