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[PATCH v6 8/8] docs/zoned-storage: add zoned device documentation

From: Sam Li
Subject: [PATCH v6 8/8] docs/zoned-storage: add zoned device documentation
Date: Fri, 5 Aug 2022 15:57:51 +0800

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

Signed-off-by: Sam Li <faithilikerun@gmail.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..c3f1e477ac
--- /dev/null
+++ b/docs/devel/zoned-storage.rst
@@ -0,0 +1,41 @@
+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
+reduces write amplification in SSDs, leading to higher throughput and increased
+capacity. More details about ZBDs can be found at:
+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 is resided 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 
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 
+  controller supports zoned storage. Use ``--blockdev zoned_host_device,
+  node-name=drive0,filename=/dev/nullb0`` to pass through ``/dev/nullb0``
+  as ``drive0``.

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