Re: [PATCH v9 7/7] docs/zoned-storage: add zoned device documentation

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 2022/09/10 14:27, Sam Li 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>
> ---
>  docs/devel/zoned-storage.rst           | 41 ++++++++++++++++++++++++++
>  docs/system/qemu-block-drivers.rst.inc |  6 ++++
>  2 files changed, 47 insertions(+)
>  create mode 100644 docs/devel/zoned-storage.rst
> 
> diff --git a/docs/devel/zoned-storage.rst b/docs/devel/zoned-storage.rst
> new file mode 100644
> index 0000000000..ead2d149cc
> --- /dev/null
> +++ b/docs/devel/zoned-storage.rst
> @@ -0,0 +1,41 @@
> +=============
> +zoned-storage
> +=============
> +
> +Zoned Block Devices (ZBDs) devide the LBA space into block regions called zones
> +that are larger than the LBA size. It can only allow sequential writes, which

s/It/They

> +reduces write amplification in SSDs, leading to higher throughput and increased
> +capacity. More details about ZBDs can be found at:

I would rephrase this like this, to be less assertive about the potential
benefits (as they depend on the vendor implementation):

..., which can reduce write amplification in SSDs, and potentially lead to
higher throughput and increased device capacity.

> +
> +https://zonedstorage.io/docs/introduction/zoned-storage
> +
> +1. Block layer APIs for zoned storage
> +-------------------------------------
> +QEMU block layer has three zoned storage model:
> +- BLK_Z_HM: This model only allows sequential writes access. It supports a set
> +of ZBD-specific I/O request that used by the host to manage device zones.
> +- BLK_Z_HA: It deals with both sequential writes and random writes access.
> +- BLK_Z_NONE: Regular block devices and drive-managed ZBDs are treated as
> +non-zoned devices.
> +
> +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, the command line for zone report testing a null_blk device of
> +qemu-io-cmds.c is:
> +$ path/to/qemu-io --image-opts 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
>  ^^^^^^^
>  

-- 
Damien Le Moal
Western Digital Research