Configuring and Starting the PostgreSQL Server

By default, PostgreSQL only accepts connections on the loopback interface. Configure PostgreSQL to accept the connections based on hostname, IP address (including CIDR address), or MAC address. A fully qualified domain name (FQDN) is not a requirement. If you do not make these changes, the services cannot connect to and use the database on which they depend.

If you are making changes to an existing database, make sure to stop any services that use the database before continuing.

Make sure that LC_ALL is set to en_US.UTF-8 and initialize the database as follows:
- RHEL 7:
```
echo 'LC_ALL="en_US.UTF-8"' >> /etc/locale.conf
sudo su -l postgres -c "postgresql-setup initdb"
```
Enable MD5 authentication. Edit pg_hba.conf, which is usually found in /var/lib/pgsql/data or /etc/postgresql/<version>/main. Add the following line:
```
host all all <range-start-ip-address>/28 md5
```
If the default pg_hba.conf file contains the following line:
```
host all all 127.0.0.1/32 ident
```
then the host line specifying md5 authentication shown above must be inserted before this ident line. Failure to do so may cause an authentication error when running the scm_prepare_database.sh script. You can modify the contents of the md5 line shown above to support different configurations. For example, if you want to access PostgreSQL from a different host, replace 127.0.0.1 with your IP address and update postgresql.conf, which is typically found in the same place as pg_hba.conf, to include:
```
listen_addresses = '*'
```
Configure settings to ensure your system performs as expected. Update these settings in the /var/lib/pgsql/data/postgresql.conf or /var/lib/postgresql/data/postgresql.conf file. Settings vary based on cluster size and resources as follows:
- Small to mid-sized clusters - Consider the following settings as starting points. If resources are limited, consider reducing the buffer sizes and checkpoint segments further. Ongoing tuning may be required based on each host's resource utilization. For example, if the Cloudera Manager Server is running on the same host as other roles, the following values may be acceptable:
  - max_connection - In general, allow each database on a host 100 maximum connections and then add 50 extra connections. You may have to increase the system resources available to PostgreSQL, as described at Connection Settings.
  - shared_buffers - 256MB
  - wal_buffers - 8MB
  - checkpoint_segments - 16
  - checkpoint_completion_target - 0.9
- Large clusters - Can contain up to 1000 hosts. Consider the following settings as starting points.
  - max_connection - For large clusters, each database is typically hosted on a different host. In general, allow each database on a host 100 maximum connections and then add 50 extra connections. You may have to increase the system resources available to PostgreSQL, as described at Connection Settings.
  - shared_buffers - 1024 MB. This requires that the operating system can allocate sufficient shared memory. See PostgreSQL information on Managing Kernel Resources for more information on setting kernel resources.
  - wal_buffers - 16 MB. This value is derived from the shared_buffers value. Setting wal_buffers to be approximately 3% of shared_buffers up to a maximum of approximately 16 MB is sufficient in most cases.
  - checkpoint_segments - 128. The PostgreSQL Tuning Guide recommends values between 32 and 256 for write-intensive systems, such as this one.
  - checkpoint_completion_target - 0.9.

Configure the PostgreSQL server to start at boot.


OS	Command
RHEL 7 compatible	`sudo systemctl enable postgresql`

Restart the PostgreSQL database:
- RHEL 7 Compatible:
```
sudo systemctl restart postgresql
```