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Document database connection limits

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docs/installation/11_optimisation.md
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@ -11,6 +11,41 @@ permalink: /installation/start/optimisation
Now that you have Dendrite running, the following tweaks will improve the reliability Now that you have Dendrite running, the following tweaks will improve the reliability
and performance of your installation. and performance of your installation.
## PostgreSQL connection limit
A PostgreSQL database engine is configured to allow only a certain number of connections.
This is typically controlled by the `max_connections` and `superuser_reserved_connections`
configuration items in `postgresql.conf`. Once these limits are violated, **PostgreSQL will
immediately stop accepting new connections** until some of the existing connections are closed.
This is a common source of misconfiguration and requires particular care.
If your PostgreSQL `max_connections` is set to `100` and `superuser_reserved_connections` is
set to `3` then you have an effective connection limit of 97 database connections. It is
therefore important to ensure that Dendrite doesn't violate that limit, otherwise database
queries will unexpectedly fail and this will cause problems both within Dendrite and for users.
If you are also running other software that uses the same PostgreSQL database engine, then you
must also take into account that some connections will be already used by your other software
and therefore will not be available to Dendrite. Check the configuration of any other software
using the same database engine for their configured connection limits and adjust your calculations
accordingly.
Dendrite has a `max_open_conns` configuration item in each `database` block to control how many
connections it will open to the database.
**If you are using the `global` database pool** then you only need to configure the
`max_open_conns` setting once in the `global` section.
**If you are defining a `database` config per component** then you will need to ensure that
the **sum total** of all configured `max_open_conns` to a given database server do not exceed
the connection limit. If you configure a total that adds up to more connections than are available
then this will cause database queries to fail.
You may wish to raise the `max_connections` limit on your PostgreSQL server to accommodate
additional connections, in which case you should also update the `max_open_conns` in your
Dendrite configuration accordingly. However be aware that this is only advisable on particularly
powerful servers that can handle the concurrent load of additional queries running at one time.
## File descriptor limit ## File descriptor limit
Most platforms have a limit on how many file descriptors a single process can open. All Most platforms have a limit on how many file descriptors a single process can open. All