expand a bit ganeti configuration, especially mentionning exporter

howto/ganeti.md

@@ -3438,13 +3438,20 @@ TODO: document gnt-job
 
 ## Interfaces
 
+TODO: document the RAPI and ssh commandline
+
 ## Authentication
 
+TODO: X509 certs and SSH
+
 ## Implementation
 
+Ganeti is implemented in a mix of Python and Haskell, in a mature
+codebase.
+
 ## Related services
 
-ref DRBD
+Ganeti relies heavily on [DRBD](howto/drbd) for live migrations.
 
 ## Issues
 
@@ -3459,17 +3466,35 @@ Upstream Ganeti has of course its own [issue tracker on GitHub](https://github.c
 
 ## Users
 
+TPA are the main direct operators of the services, but most if not all
+TPI teams use its services either directly or indirectly.
+
 ## Upstream
 
+Ganeti used to be a Google project until it was abandoned and spun off
+to a separate, standalone free software community. Right now it is
+maintained by a mixed collection of organisations and non-profits.
+
 ## Monitoring and metrics
 
-<!-- TODO: describe how this service is monitored and how it can be tested -->
-<!-- after major changes like IP address changes or upgrades -->
+Anarcat implemented a Prometheus metrics exporter that writes stats in
+the node exporter "textfile" collector. The source code is available
+in `tor-puppet.git`, as
+`profile/files/ganeti/tpa-ganeti-prometheus-metrics.py`. Those metrics
+are in turn displayed in the [Ganeti Health](https://grafana.torproject.org/d/ce2db5a5-b42b-4454-8d81-ee95b09e229a/ganeti-health) Grafana dashboard.
 
-TODO: https://github.com/ganeti/prometheus-ganeti-exporter
+The WMF worked on a [proper Ganeti exporter](https://github.com/ganeti/prometheus-ganeti-exporter) we should probably
+switch to, once it is [packaged in Debian](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1054138).
 
 ## Tests
 
+To test if a cluster is working properly, the `verify` command can be
+ran:
+
+    gnt-cluster verify
+
+Creating a VM and migrating it between machines is also a good test.
+
 ## Logs
 
 Ganeti logs a significant amount of information in
@@ -3483,7 +3508,10 @@ Ganeti logs a significant amount of information in
 
 ## Backups
 
-TODO
+There are no backups of virtual machines directly from Ganeti: each
+machine is expected to perform its own backups. The Ganeti
+configuration should be backed up as normal by our [backup
+systems](howto/backup).
 
 ## Other documentation