reflect changes in pgbackrest policies

howto/postgresql.md

@@ -2242,8 +2242,8 @@ The new backup system is based on [pgBackRest][]. It works by SSH'ing
 between the client and server and running `pgbackrest` commands, which
 encapsulates all functionality including backup, and restore.
 
-Backups are retained for 3 weeks (21 days), although the source of
-truth for this is not here but in Hiera, in `tor-puppet.git`'s
+Backups are retained for (30 days), although the source of truth for
+this is not here but in Hiera, in `tor-puppet.git`'s
 `hiera/common/postgresql.yaml`, the
 `pgbackrest::config:global:repo1-retention-full` value. Expiration is
 performed when backups are ran, from the systemd timers. See also the
@@ -2251,6 +2251,46 @@ performed when backups are ran, from the systemd timers. See also the
 
 [pgBackRest]: https://pgbackrest.org/
 
+pgBackRest considers [3 different backup types](https://pgbackrest.org/command.html#command-backup/category-command/option-type), here are schedules
+for those:
+
+| type   | frequency | note                                                                                             |
+|--------|-----------|--------------------------------------------------------------------------------------------------|
+| `full` | 30 days   | all database cluster files will be copied and there will be no dependencies on previous backups. |
+| `diff` | 7 days    | like an incremental backup but always based on the last full backup.                             |
+| `incr` | 24h       | incremental from the last successful backup.                                                     |
+
+Backups are scheduled using systemd timers exported from each node,
+based on a template per backup type, so there's a matrix of
+`pgbackrest-backup-{diff,full}@.{service,timer}` files on the
+repository server, e.g.
+
+```
+root@backup-storage-01:~# ls /etc/systemd/system | grep @\\.
+pgbackrest-backup-diff@.service
+pgbackrest-backup-diff@.timer
+pgbackrest-backup-full@.service
+pgbackrest-backup-full@.timer
+pgbackrest-backup-incr@.service
+pgbackrest-backup-incr@.timer
+```
+
+Each server has its own instance of that, a symlink to those, for
+example weather-01:
+
+```
+root@backup-storage-01:~# ls -l /etc/systemd/system | grep weather-01
+lrwxrwxrwx 1 root root   31 Dec  5 02:02 pgbackrest-backup-diff@weather-01.service -> pgbackrest-backup-diff@.service
+lrwxrwxrwx 1 root root   49 Dec  4 21:51 pgbackrest-backup-diff@weather-01.timer -> /etc/systemd/system/pgbackrest-backup-diff@.timer
+lrwxrwxrwx 1 root root   31 Dec  5 02:02 pgbackrest-backup-full@weather-01.service -> pgbackrest-backup-full@.service
+lrwxrwxrwx 1 root root   49 Dec  4 21:51 pgbackrest-backup-full@weather-01.timer -> /etc/systemd/system/pgbackrest-backup-full@.timer
+lrwxrwxrwx 1 root root   31 Dec 16 18:32 pgbackrest-backup-incr@weather-01.service -> pgbackrest-backup-incr@.service
+lrwxrwxrwx 1 root root   49 Dec 16 18:32 pgbackrest-backup-incr@weather-01.timer -> /etc/systemd/system/pgbackrest-backup-incr@.timer
+```
+
+Retention is configured at the "full" level, with the
+`repo1-retention-full` setting.
+
 ### Puppet setup
 
 PostgreSQL servers are automatically configured to use pgBackRest to