(#5849) Convert rst to markdown (#6040)

Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

docs/postgres.md

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+# Using Postgres
+
+Postgres version 9.5 or later is known to work.
+
+## Install postgres client libraries
+
+Synapse will require the python postgres client library in order to
+connect to a postgres database.
+
+-   If you are using the [matrix.org debian/ubuntu
+    packages](../INSTALL.md#matrixorg-packages), the necessary python
+    library will already be installed, but you will need to ensure the
+    low-level postgres library is installed, which you can do with
+    `apt install libpq5`.
+-   For other pre-built packages, please consult the documentation from
+    the relevant package.
+-   If you installed synapse [in a
+    virtualenv](../INSTALL.md#installing-from-source), you can install
+    the library with:
+
+        ~/synapse/env/bin/pip install matrix-synapse[postgres]
+
+    (substituting the path to your virtualenv for `~/synapse/env`, if
+    you used a different path). You will require the postgres
+    development files. These are in the `libpq-dev` package on
+    Debian-derived distributions.
+
+## Set up database
+
+Assuming your PostgreSQL database user is called `postgres`, create a
+user `synapse_user` with:
+
+    su - postgres
+    createuser --pwprompt synapse_user
+
+Before you can authenticate with the `synapse_user`, you must create a
+database that it can access. To create a database, first connect to the
+database with your database user:
+
+    su - postgres
+    psql
+
+and then run:
+
+    CREATE DATABASE synapse
+     ENCODING 'UTF8'
+     LC_COLLATE='C'
+     LC_CTYPE='C'
+     template=template0
+     OWNER synapse_user;
+
+This would create an appropriate database named `synapse` owned by the
+`synapse_user` user (which must already have been created as above).
+
+Note that the PostgreSQL database *must* have the correct encoding set
+(as shown above), otherwise it will not be able to store UTF8 strings.
+
+You may need to enable password authentication so `synapse_user` can
+connect to the database. See
+<https://www.postgresql.org/docs/11/auth-pg-hba-conf.html>.
+
+## Tuning Postgres
+
+The default settings should be fine for most deployments. For larger
+scale deployments tuning some of the settings is recommended, details of
+which can be found at
+<https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server>.
+
+In particular, we've found tuning the following values helpful for
+performance:
+
+-   `shared_buffers`
+-   `effective_cache_size`
+-   `work_mem`
+-   `maintenance_work_mem`
+-   `autovacuum_work_mem`
+
+Note that the appropriate values for those fields depend on the amount
+of free memory the database host has available.
+
+## Synapse config
+
+When you are ready to start using PostgreSQL, edit the `database`
+section in your config file to match the following lines:
+
+    database:
+        name: psycopg2
+        args:
+            user: <user>
+            password: <pass>
+            database: <db>
+            host: <host>
+            cp_min: 5
+            cp_max: 10
+
+All key, values in `args` are passed to the `psycopg2.connect(..)`
+function, except keys beginning with `cp_`, which are consumed by the
+twisted adbapi connection pool.
+
+## Porting from SQLite
+
+### Overview
+
+The script `synapse_port_db` allows porting an existing synapse server
+backed by SQLite to using PostgreSQL. This is done in as a two phase
+process:
+
+1.  Copy the existing SQLite database to a separate location (while the
+    server is down) and running the port script against that offline
+    database.
+2.  Shut down the server. Rerun the port script to port any data that
+    has come in since taking the first snapshot. Restart server against
+    the PostgreSQL database.
+
+The port script is designed to be run repeatedly against newer snapshots
+of the SQLite database file. This makes it safe to repeat step 1 if
+there was a delay between taking the previous snapshot and being ready
+to do step 2.
+
+It is safe to at any time kill the port script and restart it.
+
+### Using the port script
+
+Firstly, shut down the currently running synapse server and copy its
+database file (typically `homeserver.db`) to another location. Once the
+copy is complete, restart synapse. For instance:
+
+    ./synctl stop
+    cp homeserver.db homeserver.db.snapshot
+    ./synctl start
+
+Copy the old config file into a new config file:
+
+    cp homeserver.yaml homeserver-postgres.yaml
+
+Edit the database section as described in the section *Synapse config*
+above and with the SQLite snapshot located at `homeserver.db.snapshot`
+simply run:
+
+    synapse_port_db --sqlite-database homeserver.db.snapshot \
+        --postgres-config homeserver-postgres.yaml
+
+The flag `--curses` displays a coloured curses progress UI.
+
+If the script took a long time to complete, or time has otherwise passed
+since the original snapshot was taken, repeat the previous steps with a
+newer snapshot.
+
+To complete the conversion shut down the synapse server and run the port
+script one last time, e.g. if the SQLite database is at `homeserver.db`
+run:
+
+    synapse_port_db --sqlite-database homeserver.db \
+        --postgres-config homeserver-postgres.yaml
+
+Once that has completed, change the synapse config to point at the
+PostgreSQL database configuration file `homeserver-postgres.yaml`:
+
+    ./synctl stop
+    mv homeserver.yaml homeserver-old-sqlite.yaml
+    mv homeserver-postgres.yaml homeserver.yaml
+    ./synctl start
+
+Synapse should now be running against PostgreSQL.