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[PATCH v16 8/8] docs/zoned-storage: add zoned device documentation

From: Sam Li
Subject: [PATCH v16 8/8] docs/zoned-storage: add zoned device documentation
Date: Fri, 10 Mar 2023 18:24:03 +0800

Add the documentation about the zoned device support to virtio-blk

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

diff --git a/docs/devel/zoned-storage.rst b/docs/devel/zoned-storage.rst
new file mode 100644
index 0000000000..6a36133e51
--- /dev/null
+++ b/docs/devel/zoned-storage.rst
@@ -0,0 +1,43 @@
+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:
+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
+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=host_device,filename=/dev/nullb0
+-c "zrp offset nr_zones"
diff --git a/docs/system/qemu-block-drivers.rst.inc 
index dfe5d2293d..105cb9679c 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 
+  controller supports zoned storage. Use ``--blockdev host_device,
+  node-name=drive0,filename=/dev/nullb0,cache.direct=on`` to pass through
+  ``/dev/nullb0`` as ``drive0``.

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