howto/drbd.md

@@ -110,6 +110,23 @@ Then that device map can be removed with:
 
     # dmsetup remove dm-34
 
+## Deleting a device after it was manually detached
+
+After manually detaching a disk from a Ganeti instance, Prometheus alerts
+something like this: "DRBD has 2 out of date disks on
+dal-node-01.torproject.org". If you **really** don't need that disk anymore,
+you can manually delete it from drbd.
+
+First, [query
+Prometheus](https://prometheus.torproject.org/classic/graph?g0.range_input=1h&g0.expr=node_drbd_disk_state_is_up_to_date%3D%3D1&g0.tab=1)
+to learn the device number. In my case, `device="drbd34"`.
+
+After making sure that that device really corresponds to the one you want to
+delete, run:
+
+    drbdsetup detach --force=yes 34
+    drbdsetup down resource34
+
 ## Pager playbook
 
 ### Resyncing disks
@@ -148,8 +165,9 @@ Then you can "reactivate" the disks simply by telling ganeti:
 
 And then the disk will resync.
 
-It's also possible a disk was detached and improperly removed. In that
-case, you might want to [delete a stray device](#deleting-a-stray-device).
+It's also possible a disk was detached and improperly removed. In that case,
+you might want to [delete a device after it was manually
+detached](#deleting-a-device-after-it-was-manually-detached).
 
 ## Upstream documentation