Document volume server maintenance mode for https://github.com/seaweedfs/seaweedfs/issues/7977

Volume-Management.md

@@ -66,3 +66,26 @@ The balancing plan will try to evenly spread the number of writable and readonly
 Run `weed shell` and `volume.mount -node <host>:<port> -volumeId <id>` to mount a volume file.
 
 To mount all new volume files you can send a hang-up signal to the volume server causing a reload with a command such as `pkill -HUP -f "weed volume"`.
+
+## Servicing live volumes
+
+When dealing with hardware storage issues, it can be useful to prevent writes to volume servers _without_ stopping the service altogether - f.ex. on volumes with RAID storage backends. Volume servers support a maintenance mode for this: when enabled, the server becomes read-only. Reads will succeed, but any write attempt will error out.
+
+Maintenance mode can be managed via the `volumeServer.state` shell command:
+
+```
+> volumeServer.state
+192.168.10.111:9007	 -> Maintenance mode: no
+192.168.10.111:9008	 -> Maintenance mode: no
+192.168.10.111:9009	 -> Maintenance mode: no
+
+> volumeServer.state --nodes 192.168.10.111:9009 --maintenanceOn
+192.168.10.111:9009	 -> Maintenance mode: yes
+
+> volumeServer.state
+192.168.10.111:9007	 -> Maintenance mode: no
+192.168.10.111:9008	 -> Maintenance mode: no
+192.168.10.111:9009	 -> Maintenance mode: yes
+```
+
+Maintenance mode is a sticky server state flag. Changes are effective immediately, and will persist even if the server is restarted.