doc/07-Operations.md: New Operations section

The newly introduced Operations section is a first attempt at an
operational Icinga DB documentation.

It covers
- essential Icinga DB monitoring,
- backups and corner cases for MySQL/MariaDB,
- MySQL/MariaDB configuration options, including AWS RDS and Galera,
- and memory overcommitment for Redis.

As such a section can never be completed, this documentation is a target
for continuous improvement.

Fixes #745.

doc/07-Operations.md

@@ -0,0 +1,98 @@
+# Operations
+
+This section is a loose collection of various topics and external references for running Icinga DB on a day-to-day basis.
+It covers topics such as self-monitoring, backups, and specifics of third-party components.
+
+## Monitor Icinga DB
+
+It is strongly recommended to monitor the monitoring.
+
+There is a built-in [`icingadb` check command](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#icingadb) in the Icinga 2 ITL.
+It covers several potential errors, including operations that take too long or invalid high availability scenarios.
+Even if the Icinga DB has crashed, checks will still run and Icinga 2 would generate notifications.
+
+In addition, both the Redis® and the relational database should be monitored.
+There are predefined check commands in the ITL to choose from.
+
+- [`redis`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#redis)
+- [`mysql`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#mysql)
+- [`mysql_health`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#mysql_health)
+- [`postgres`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#postgres)
+
+A simpler approach would be to check if the processes are running, e.g.,
+with [`proc`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#procs) or
+[`systemd`](https://icinga.com/docs/icinga-2/latest/doc/10-icinga-template-library/#systemd).
+
+## Backups
+
+There are only two things to back up in Icinga DB.
+
+1. The configuration file in `/etc/icingadb` and
+2. the relational database, using `mysqldump`, `mariadb-dump` or `pg_dump`.
+
+!!! warning
+
+    When creating a database dump for MySQL or MariaDB with `mysqldump` or `mariadb-dump`,
+    use the [`--single-transaction` command line argument flag](https://dev.mysql.com/doc/refman/8.4/en/mysqldump.html#option_mysqldump_single-transaction)
+    to not lock the whole database while the backup is running.
+
+## Third-Party Configuration
+
+Icinga DB relies on external components to work.
+The following collection is based on experience.
+It is a target for continuous improvement.
+
+### MySQL and MariaDB
+
+#### `max_allow_packets`
+
+The `max_allow_packets` system variable limits the size of messages between MySQL/MariaDB servers and clients.
+More information is available in
+[MySQL's "Replication and max_allowed_packet" documentation section](https://dev.mysql.com/doc/refman/8.4/en/replication-features-max-allowed-packet.html),
+[MySQL's variable documentation](https://dev.mysql.com/doc/refman/8.4/en/server-system-variables.html#sysvar_max_allowed_packet) and
+[MariaDB's variable documentation](https://mariadb.com/kb/en/server-system-variables/#max_allowed_packet).
+
+The database configuration should have `max_allow_packets` set to at least `64M`.
+
+#### Amazon RDS for MySQL
+
+When importing the MySQL schema into Amazon RDS for MySQL, the following may occur.
+
+```
+Error 1419: You do not have the SUPER privilege and binary logging is enabled (you *might* want to use the less safe log_bin_trust_function_creators variable)
+```
+
+This error can be mitigated by creating and modifying a custom DB parameter group as described in the related [AWS Knowledge Center article](https://repost.aws/knowledge-center/rds-mysql-functions).
+
+#### Galera Cluster
+
+Starting with Icinga DB version 1.2.0, Galera support has been added to the Icinga DB daemon.
+Its specific database configuration is described in the [Galera configuration section](03-Configuration.md#galera-cluster).
+
+As mentioned in [MariaDB's known Galera cluster limitations](https://mariadb.com/kb/en/mariadb-galera-cluster-known-limitations/),
+transactions are limited in both amount of rows (128K) and size (2GiB).
+A busy Icinga setup can cause Icinga DB to create transactions that exceed these limits with the default configuration.
+
+If you get an error like `Error 1105 (HY000): Maximum writeset size exceeded`
+and your Galera node logs something like `WSREP: transaction size limit (2147483647) exceeded`,
+decrease the values of `max_placeholders_per_statement` and `max_rows_per_transaction` in Icinga DB's
+[Database Options](https://icinga.com/docs/icinga-db/latest/doc/03-Configuration/#database-options).
+
+### Redis®
+
+On Linux, enable [memory overcommitting](https://www.kernel.org/doc/Documentation/vm/overcommit-accounting).
+
+```shell
+sysctl vm.overcommit_memory=1
+```
+
+To persist this setting across reboots, add the following line to [`sysctl.conf(5)`](https://man7.org/linux/man-pages/man5/sysctl.conf.5.html).
+If your distribution uses systemd, a configuration file under `/etc/sysctl.d/` is required, as described by
+[`systemd-sysctl.service(8)`](https://www.freedesktop.org/software/systemd/man/latest/systemd-sysctl.service.html) and
+[`sysctl.d(5)`](https://man7.org/linux/man-pages/man5/sysctl.d.5.html).
+
+```
+vm.overcommit_memory = 1
+```
+
+In addition, the official [Redis® administration documentation](https://redis.io/docs/latest/operate/oss_and_stack/management/admin/) is quite useful.