docs/zoned-storage: add zoned device documentation

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>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20230508045533.175575-9-faithilikerun@gmail.com
Message-id: 20230324090605.28361-9-faithilikerun@gmail.com
[Add index-api.rst to fix "zoned-storage.rst:document isn't included in
any toctree" error and fix pre-formatted code syntax.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

2 files changed, 44 insertions, 0 deletions

diff --git a/docs/devel/index-api.rst b/docs/devel/index-api.rst
index 60c0d74..7108821 100644
--- a/docs/devel/index-api.rst
+++ b/docs/devel/index-api.rst
@@ -12,3 +12,4 @@ generated from in-code annotations to function prototypes.
    memory
    modules
    ui
+   zoned-storage
diff --git a/docs/devel/zoned-storage.rst b/docs/devel/zoned-storage.rst
new file mode 100644
index 0000000..da78db2
--- /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=host_device,filename=/dev/nullb0 -c "zrp offset nr_zones"