jsonrpc.md: document NBD RPC methods

Change-Id: Ia4ea6986c2282b3a56ec48ca4f9b0ad05e95e7f1
Signed-off-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/450578
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/jsonrpc.md

@@ -5115,6 +5115,133 @@ Example response:
 }
 ~~~
 
+# Linux Network Block Device (NBD) {#jsonrpc_components_nbd}
+
+SPDK supports exporting bdevs through Linux nbd. These devices then appear as standard Linux kernel block devices and can be accessed using standard utilities like fdisk.
+
+In order to export a device over nbd, first make sure the Linux kernel nbd driver is loaded by running 'modprobe nbd'.
+
+## start_nbd_disk {#rpc_start_nbd_disk}
+
+Start to export one SPDK bdev as NBD disk
+
+### Parameters
+
+Name                    | Optional | Type        | Description
+----------------------- | -------- | ----------- | -----------
+bdev_name               | Required | string      | Bdev name to export
+nbd_device              | Optional | string      | NBD device name to assign
+
+### Response
+
+Path of exported NBD disk
+
+### Example
+
+Example request:
+
+~~~
+{
+ "params": {
+    "nbd_device": "/dev/nbd1",
+    "bdev_name": "Malloc1"
+  },
+  "jsonrpc": "2.0",
+  "method": "start_nbd_disk",
+  "id": 1
+}
+~~~
+
+Example response:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result": "/dev/nbd1"
+}
+~~~
+
+## stop_nbd_disk {#rpc_stop_nbd_disk}
+
+Stop one NBD disk which is based on SPDK bdev.
+
+### Parameters
+
+Name                    | Optional | Type        | Description
+----------------------- | -------- | ----------- | -----------
+nbd_device              | Required | string      | NBD device name to stop
+
+### Example
+
+Example request:
+
+~~~
+{
+ "params": {
+    "nbd_device": "/dev/nbd1",
+  },
+  "jsonrpc": "2.0",
+  "method": "stop_nbd_disk",
+  "id": 1
+}
+~~~
+
+Example response:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result": "true"
+}
+~~~
+
+## get_nbd_disks {#rpc_get_nbd_disks}
+
+Display all or specified NBD device list
+
+### Parameters
+
+Name                    | Optional | Type        | Description
+----------------------- | -------- | ----------- | -----------
+nbd_device              | Optional | string      | NBD device name to display
+
+### Response
+
+The response is an array of exported NBD devices and their corresponding SPDK bdev.
+
+### Example
+
+Example request:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "method": "get_nbd_disks",
+  "id": 1
+}
+~~~
+
+Example response:
+
+~~~
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result":  [
+    {
+      "bdev_name": "Malloc0",
+      "nbd_device": "/dev/nbd0"
+    },
+    {
+      "bdev_name": "Malloc1",
+      "nbd_device": "/dev/nbd1"
+    }
+  ]
+}
+~~~
+
 # Miscellaneous RPC commands
 
 ## send_nvme_cmd {#rpc_send_nvme_cmd}