16.3. Starting the Database Server

Before anyone can access the database, you must start the database server. The database server program is called postgres . The postgres program must know where to find the data it is supposed to use. This is done with the -D option. Thus, the simplest way to start the server is:

$ postgres -D /usr/local/pgsql/data

which will leave the server running in the foreground. This must be done while logged into the PostgreSQL user account. Without -D , the server will try to use the data directory named by the environment variable PGDATA . If that variable is not provided either, it will fail.

Normally it is better to start postgres in the background. For this, use the usual shell syntax:

$ postgres -D /usr/local/pgsql/data >logfile 2>&1 &

It is important to store the server's stdout and stderr output somewhere, as shown above. It will help for auditing purposes and to diagnose problems. (See Section 22.3 for a more thorough discussion of log file handling.)

The postgres program also takes a number of other command-line options. For more information, see the postgres reference page and Chapter 17 below.

This shell syntax can get tedious quickly. Therefore the wrapper program pg_ctl is provided to simplify some tasks. For example:

pg_ctl start -l logfile

will start the server in the background and put the output into the named log file. The -D option has the same meaning here as for postgres . pg_ctl is also capable of stopping the server.

Normally, you will want to start the database server when the computer boots. Autostart scripts are operating-system-specific. There are a few distributed with PostgreSQL in the contrib/start-scripts directory. Installing one will require root privileges.

Different systems have different conventions for starting up daemons at boot time. Many systems have a file /etc/rc.local or /etc/rc.d/rc.local . Others use rc.d directories. Whatever you do, the server must be run by the PostgreSQL user account and not by root or any other user. Therefore you probably should form your commands using su -c '...' postgres . For example:

su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres

Here are a few more operating-system-specific suggestions. (In each case be sure to use the proper installation directory and user name where we show generic values.)

While the server is running, its PID is stored in the file postmaster.pid in the data directory. This is used to prevent multiple server instances from running in the same data directory and can also be used for shutting down the server.

16.3.1. Server Start-up Failures

There are several common reasons the server might fail to start. Check the server's log file, or start it by hand (without redirecting standard output or standard error) and see what error messages appear. Below we explain some of the most common error messages in more detail.

LOG: could not bind IPv4 socket: Address already in use HINT: Is another postmaster already running on port 5432? If not, wait a few seconds and retry. FATAL: could not create TCP/IP listen socket

This usually means just what it suggests: you tried to start another server on the same port where one is already running. However, if the kernel error message is not Address already in use or some variant of that, there may be a different problem. For example, trying to start a server on a reserved port number may draw something like:

$ postgres -p 666 LOG: could not bind IPv4 socket: Permission denied HINT: Is another postmaster already running on port 666? If not, wait a few seconds and retry. FATAL: could not create TCP/IP listen socket

A message like

FATAL: could not create shared memory segment: Invalid argument DETAIL: Failed system call was shmget(key=5440001, size=4011376640, 03600).

probably means your kernel's limit on the size of shared memory is smaller than the work area PostgreSQL is trying to create (4011376640 bytes in this example). Or it could mean that you do not have System-V-style shared memory support configured into your kernel at all. As a temporary workaround, you can try starting the server with a smaller-than-normal number of buffers ( shared_buffers ). You will eventually want to reconfigure your kernel to increase the allowed shared memory size. You may also see this message when trying to start multiple servers on the same machine, if their total space requested exceeds the kernel limit.

An error like

FATAL: could not create semaphores: No space left on device DETAIL: Failed system call was semget(5440126, 17, 03600).

does not mean you've run out of disk space. It means your kernel's limit on the number of System V semaphores is smaller than the number PostgreSQL wants to create. As above, you may be able to work around the problem by starting the server with a reduced number of allowed connections ( max_connections ), but you'll eventually want to increase the kernel limit.

If you get an "illegal system call" error, it is likely that shared memory or semaphores are not supported in your kernel at all. In that case your only option is to reconfigure the kernel to enable these features.

Details about configuring System V IPC facilities are given in Section 16.4.1 .

16.3.2. Client Connection Problems

Although the error conditions possible on the client side are quite varied and application-dependent, a few of them might be directly related to how the server was started up. Conditions other than those shown below should be documented with the respective client application.

psql: could not connect to server: Connection refused Is the server running on host "server.joe.com" and accepting TCP/IP connections on port 5432?

This is the generic "I couldn't find a server to talk to" failure. It looks like the above when TCP/IP communication is attempted. A common mistake is to forget to configure the server to allow TCP/IP connections.

Alternatively, you'll get this when attempting Unix-domain socket communication to a local server:

psql: could not connect to server: No such file or directory Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

The last line is useful in verifying that the client is trying to connect to the right place. If there is in fact no server running there, the kernel error message will typically be either Connection refused or No such file or directory , as illustrated. (It is important to realize that Connection refused in this context does not mean that the server got your connection request and rejected it. That case will produce a different message, as shown in Section 20.3 .) Other error messages such as Connection timed out may indicate more fundamental problems, like lack of network connectivity.