[RFC v5 11/11] docs/zoned-storage: add zoned device documentation

[Sam Li]

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

Signed-off-by: Sam Li <faithilikerun@gmail.com>
---
 docs/devel/zoned-storage.rst           | 68 ++++++++++++++++++++++++++
 docs/system/qemu-block-drivers.rst.inc |  6 +++
 2 files changed, 74 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..e62927dceb
--- /dev/null
+++ b/docs/devel/zoned-storage.rst
@@ -0,0 +1,68 @@
+=============
+zoned-storage
+=============
+
+Zoned Block Devices (ZBDs) devide the LBA space to block regions called zones
+that are larger than the LBA size. It can only allow sequential writes, which
+reduces write amplification in SSD, leading to higher throughput and increased
+capacity. More details about ZBDs can be found at:
+
+https://zonedstorage.io/docs/introduction/zoned-storage
+
+zone emulation
+--------------
+In its current status, the virtio-blk device is not aware of ZBDs but the guest
+sees host-managed drives as regular drive that will runs correctly under the
+most common write workloads.
+
+The zoned device support aims to let guests (virtual machines) access zoned
+storage devices on the host (hypervisor) through a virtio-blk device. This
+involves extending QEMU's block layer and virtio-blk emulation code.
+
+If the host supports zoned block devices, it can set VIRTIO_BLK_F_ZONED. Then
+in the guest side, it appears following situations:
+1) If the guest virtio-blk driver sees the VIRTIO_BLK_F_ZONED bit set, then it
+will assume that the zoned characteristics fields of the config space are 
valid.
+2) If the guest virtio-blk driver sees a zoned model that is NONE, then it is
+known that is a regular block device.
+3) If the guest virtio-blk driver sees a zoned model that is HM(or HA), then it
+is known that is a zoned block device and probes the other zone fields.
+
+On QEMU sides,
+1) The DEFINE PROP BIT macro must be used to declare that the host supports
+zones.
+2) BlockDrivers can declare zoned device support once known the zoned model
+for the block device is not NONE.
+
+zoned storage APIs
+------------------
+
+Zone emulation part extends the block layer APIs and virtio-blk emulation 
section
+with the minimum set of zoned commands that are necessary to support zoned
+devices. The commands are - Report Zones, four zone operations and Zone Append
+(developing).
+
+testing
+-------
+
+It can be tested on a null_blk device using qemu-io, qemu-iotests or blkzone(8)
+command in the guest os.
+
+1. For example, the command line for zone report using qemu-io is:
+
+$ path/to/qemu-io --image-opts driver=zoned_host_device,filename=/dev/nullb0 -c
+"zrp offset nr_zones"
+
+To enable zoned device in the guest os, the guest kernel must have the 
virtio-blk
+driver with ZBDs support. The link to such patches for the kernel is:
+
+https://github.com/dmitry-fomichev/virtblk-zbd
+
+Then, add the following options to the QEMU command line:
+-blockdev node-name=drive0,driver=zoned_host_device,filename=/dev/nullb0
+
+After the guest os booting, use blkzone(8) to test zone operations:
+blkzone report -o offset -c nr_zones /dev/vda
+
+2. We can also use the qemu-iotests in ./tests/qemu-iotests/tests/zoned.sh.
+
diff --git a/docs/system/qemu-block-drivers.rst.inc 
b/docs/system/qemu-block-drivers.rst.inc
index dfe5d2293d..2a761a4b80 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 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
 ^^^^^^^
 
-- 
2.37.1