start documenting cluster migration procedures

howto/ganeti.md

@@ -2056,12 +2056,56 @@ Look into logs on the relevant nodes (particularly
 `/var/log/ganeti/node-daemon.log`, which shows all commands ran by
 ganeti) when you have problems.
 
+### Migrating a VM between clusters
+
+The [export/import](https://docs.ganeti.org/docs/ganeti/3.0/html/admin.html#export-import) mechanism can also be used to export and import
+VMs one at a time, if only a subset of the cluster needs to be
+evacuated.
+
+ 1. find nodes to host the exported VM on the source cluster and the
+    target cluster; it needs enough disk space in `/srv/ganeti` to
+    keep a copy of a snapshot of the VM:
+
+        df -h /srv/ganeti
+
+ 2. have the right kernel modules loaded, which might require a
+    reboot of the source node:
+
+        modprobe dm_snapshot
+
+ 3. on the master of the source Ganeti cluster, export the VM to the
+    source node, also use `--noshutdown` if you cannot afford to have
+    downtime on the VM *and* you are ready to lose data accumulated
+    after the snapshot:
+
+        gnt-backup export -n fsn-node-01.torproject.org test-01.torproject.org
+
+ 4. copy the VM snapshot from the source node to node in the target
+    cluster:
+
+        scp /srv/ganeti/... root@chi-node-02.torproject.org:/srv/ganeti
+
+ 5. on the master of the target Ganeti cluster, import the VM:
+
+        gnt-backup import -n chi-node-08.torproject.org --src-node=chi-node-02.torproject.org --src-dir=/srv/ganeti/??? test-01.torproject.org
+
+### Mass migrating instances to a new cluster
+
+The [move-instance](https://docs.ganeti.org/docs/ganeti/3.0/html/move-instance.html) command can do this.
+
+TODO: document mass cluster migrations.
+
 ## Disaster recovery
 
 If things get completely out of hand and the cluster becomes too
 unreliable for service, the only solution is to rebuild another one
-elsewhere. Since Ganeti 2.2, there is a [move-instance](http://docs.ganeti.org/ganeti/2.15/html/move-instance.html) command to
-move instances between cluster that can be used for that purpose.
+elsewhere. Since Ganeti 2.2, there is a [move-instance](https://docs.ganeti.org/docs/ganeti/3.0/html/move-instance.html) command to
+move instances between cluster that can be used for that purpose. See
+the [mass migration procedure](#mass-migrating-instances-to-a-new-cluster) above.
+
+The [export/import](https://docs.ganeti.org/docs/ganeti/3.0/html/admin.html#export-import) mechanism can also be used to export and import
+VMs one at a time, if only a subset of the cluster needs to be
+evacuated. See the [migrating a VM between clusters](#migrating-a-vm-between-clusters) procedure above.
 
 If Ganeti is completely destroyed and its APIs don't work anymore, the
 last resort is to restore all virtual machines from