Move NBD netlink map&unmap to separate commands, add "netlink-revive" command

docs/usage/nbd.en.md

@@ -15,12 +15,21 @@ See also [VDUSE](qemu.en.md#vduse) as a better alternative to NBD.
 
 Vitastor Kubernetes CSI driver uses NBD when VDUSE is unavailable.
 
-## Map image
+Supports the following commands:
+
+- [map](#map)
+- [unmap](#unmap)
+- [ls](#ls)
+- [netlink-map](#netlink-map)
+- [netlink-unmap](#netlink-unmap)
+- [netlink-revive](#netlink-revive)
+
+## map
 
 To create a local block device for a Vitastor image run:
 
 ```
-vitastor-nbd map --image testimg
+vitastor-nbd map [/dev/nbdN] --image testimg
 ```
 
 It will output a block device name like /dev/nbd0 which you can then use as a normal disk.
@@ -29,25 +38,25 @@ You can also use `--pool <POOL> --inode <INODE> --size <SIZE>` instead of `--ima
 
 vitastor-nbd supports all usual Vitastor configuration options like `--config_file <path_to_config>` plus NBD-specific:
 
-* `--nbd_timeout 300` \
-  Timeout for I/O operations in seconds after exceeding which the kernel stops
-  the device. You can set it to 0 to disable the timeout, but beware that you
-  won't be able to stop the device at all if vitastor-nbd process dies.
+* `--nbd_timeout 0` \
+  Timeout for I/O operations in seconds after exceeding which the kernel stops the device.
+  Before Linux 5.19, if nbd_timeout is 0, a dead NBD device can't be removed from
+  the system at all without rebooting.
 * `--nbd_max_devices 64 --nbd_max_part 3` \
   Options for the `nbd` kernel module when modprobing it (`nbds_max` and `max_part`).
-  note that maximum allowed (nbds_max)*(1+max_part) is 256.
 * `--logfile /path/to/log/file.txt` \
   Write log messages to the specified file instead of dropping them (in background mode)
   or printing them to the standard output (in foreground mode).
 * `--dev_num N` \
-  Use the specified device /dev/nbdN instead of automatic selection.
+  Use the specified device /dev/nbdN instead of automatic selection (alternative syntax
+  to /dev/nbdN positional parameter).
 * `--foreground 1` \
   Stay in foreground, do not daemonize.
 
 Note that `nbd_timeout`, `nbd_max_devices` and `nbd_max_part` options may also be specified
 in `/etc/vitastor/vitastor.conf` or in other configuration file specified with `--config_file`.
 
-## Unmap image
+## unmap
 
 To unmap the device run:
 
@@ -55,12 +64,14 @@ To unmap the device run:
 vitastor-nbd unmap /dev/nbd0
 ```
 
-## List mapped images
+## ls
 
 ```
 vitastor-nbd ls [--json]
 ```
 
+List mapped images.
+
 Example output (normal format):
 
 ```
@@ -78,3 +89,45 @@ Example output (JSON format):
 ```
 {"/dev/nbd0": {"image": "bench", "pid": 584536}, "/dev/nbd1": {"image": "bench1", "pid": 584546}}
 ```
+
+## netlink-map
+
+```
+vitastor-nbd netlink-map [/dev/nbdN] (--image <image> | --pool <pool> --inode <inode> --size <size in bytes>)
+```
+
+On recent kernel versions it's also possinle to map NBD devices using netlink interface.
+
+This is an experimental feature because it doesn't solve all issues of NBD. Differences from regular ioctl-based 'map':
+
+1. netlink-map can create new `/dev/nbdN` devices (those not present in /dev/).
+2. netlink-mapped devices can be unmapped only using `netlink-unmap` command.
+3. netlink-mapped devices don't show up `ls` output (yet).
+4. Dead netlink-mapped devices can be 'revived' using `netlink-revive`.
+   However, old I/O requests will hang forever if `nbd_timeout` is not specified.
+5. netlink-map supports additional options:
+
+* `--nbd_conn_timeout 0` \
+  Disconnect a dead device automatically after this number of seconds.
+* `--nbd_destroy_on_disconnect 1` \
+  Delete the nbd device on disconnect.
+* `--nbd_disconnect_on_close 1` \
+  Disconnect the nbd device on close by last opener.
+* `--nbd_ro 1` \
+  Set device into read only mode.
+
+## netlink-unmap
+
+```
+vitastor-nbd netlink-unmap /dev/nbdN
+```
+
+Unmap a device using netlink interface. Works with both netlink and ioctl mapped devices.
+
+## netlink-revive
+
+```
+vitastor-nbd netlink-revive /dev/nbdX (--image <image> | --pool <pool> --inode <inode> --size <size in bytes>)
+```
+
+Restart a dead NBD netlink-mapped device without removing it. Supports the same options as `netlink-map`.