postmaster is the PostgreSQL multiuser database server. In order for a client application to access a database it connects (over a network or locally) to a running postmaster. The postmaster then starts a separate server process ("postgres") to handle the connection. The postmaster also manages the communication among server processes.
By default the postmaster starts in the foreground and prints log messages to the standard output. In practical applications the postmaster should be started as a background process, perhaps at boot time.
One postmaster always manages the data from exactly one database cluster. A database cluster is a collection of databases that is stored at a common file system location. When the postmaster starts it needs to know the location of the database cluster files ("data area"). This is done with the -D invocation option or the PGDATA environment variable; there is no default. More than one postmaster process can run on a system at one time, as long as they use different data areas and different communication ports (see below). A data area is created with initdb.
postmaster accepts the following command line arguments. For a detailed discussion of the options consult the Administrator's Guide. You can also save typing most of these options by setting up a configuration file.
Enables run-time assert checks, which is a debugging aid to detect programming mistakes. This is only available if it was enabled during compilation. If so, the default is on.
Sets the number of shared buffers for use by the server processes. This value defaults to 64 buffers, where each buffer is 8 kB.
Sets a named run-time parameter. Consult the Administrator's Guide for a list and descriptions. Most of the other command line options are in fact short forms of such a parameter assignment. -c can appear multiple times to set multiple parameters.
Sets the debug level. The higher this value is set, the more debugging output is written to the server log. The default is 0, which means no debugging. Values up to 4 are useful; higher numbers produce no additional output.
Specifies the file system location of the data directory. See discussion above.
Disables fsync calls for performance improvement, at the risk of data corruption in event of a system crash. Read the detailed documentation before using this!
Specifies the TCP/IP host name or address on which the postmaster is to listen for connections from client applications. Defaults to listening on all configured addresses (including localhost).
Allows clients to connect via TCP/IP (Internet domain) connections. Without this option, only local Unix domain socket connections are accepted.
Specifies the directory of the Unix-domain socket on which the postmaster is to listen for connections from client applications. The default is normally /tmp, but can be changed at build time.
Enables secure connections using SSL. The -i option is also required. You must have compiled with SSL enabled to use this option.
Sets the maximum number of client connections that this postmaster will accept. By default, this value is 32, but it can be set as high as your system will support. (Note that -B is required to be at least twice -N. See the Administrator's Guide for a discussion of system resource requirements for large numbers of client connections.)
The command line-style options specified in extra-options are passed to all backend server processes started by this postmaster. See postgres for possibilities. If the option string contains any spaces, the entire string must be quoted.
Specifies the TCP/IP port or local Unix domain socket file extension on which the postmaster is to listen for connections from client applications. Defaults to the value of the PGPORT environment variable, or if PGPORT is not set, then defaults to the value established during compilation (normally 5432). If you specify a port other than the default port, then all client applications must specify the same port using either command-line options or PGPORT.
Specifies that the postmaster process should start up in silent mode. That is, it will disassociate from the user's (controlling) terminal, start its own process group, and redirect its standard output and standard error to /dev/null.
Using this switch discards all logging output, which is probably not what you want, since it makes it very difficult to troubleshoot problems. See below for a better way to start the postmaster in the background.
Sets a named run-time parameter; a shorter form of -c.
Two additional command line options are available for debugging problems that cause a backend to die abnormally. These options control the behavior of the postmaster in this situation, and neither option is intended for use in ordinary operation.
The ordinary strategy for this situation is to notify all other backends that they must terminate and then reinitialize the shared memory and semaphores. This is because an errant backend could have corrupted some shared state before terminating.
These special-case options are:
postmaster will not reinitialize shared data structures. A knowledgeable system programmer can then use a debugger to examine shared memory and semaphore state.
postmaster will stop all other backend processes by sending the signal SIGSTOP, but will not cause them to terminate. This permits system programmers to collect core dumps from all backend processes by hand.
If you see this message, you should run the ipcclean command. After doing so, try starting postmaster again. If this still doesn't work, you probably need to configure your kernel for shared memory and semaphores as described in the installation notes. If you run multiple instances of postmaster on a single host, or have a kernel with particularly small shared memory and/or semaphore limits, you may have to reconfigure your kernel to increase its shared memory or semaphore parameters.
Tip: You may be able to postpone reconfiguring your kernel by decreasing -B to reduce the shared memory consumption of PostgreSQL, and/or by reducing -N to reduce the semaphore consumption.
If you see this message, you should make certain that there is no other postmaster process already running on the same port number. The easiest way to determine this is by using the command
$ ps ax | grep postmaster
or
$ ps -e | grep postmaster
depending on your system.
If you are sure that no other postmaster processes are running and you still get this error, try specifying a different port using the -p option. You may also get this error if you terminate the postmaster and immediately restart it using the same port; in this case, you must simply wait a few seconds until the operating system closes the port before trying again. Finally, you may get this error if you specify a port number that your operating system considers to be reserved. For example, many versions of Unix consider port numbers under 1024 to be trusted and only permit the Unix superuser to access them.
If at all possible, do not use SIGKILL to kill the postmaster. This will prevent postmaster from freeing the system resources (e.g., shared memory and semaphores) that it holds before terminating.
To terminate the postmaster normally, the signals SIGTERM, SIGINT, or SIGQUIT can be used. The first will wait for all clients to terminate before quitting, the second will forcefully disconnect all clients, and the third will quit immediately without proper shutdown, resulting in a recovery run during restart.
The utility command pg_ctl can be used to start and shut down the postmaster safely and comfortably.
The -- options will not work on FreeBSD or OpenBSD. Use -c instead. This is a bug in the affected operating systems; a future release of PostgreSQL will provide a workaround if this is not fixed.
To start postmaster in the background using default values, type:
$ nohup postmaster >logfile 2>&1 </dev/null &
To start postmaster with a specific port:
$ postmaster -p 1234
This command will start up postmaster communicating through the port 1234. In order to connect to this postmaster using psql, you would need to run it as
$ psql -p 1234
or set the environment variable PGPORT:
$ export PGPORT=1234 $ psql
Named runtime parameters can be set in either of these styles:
$ postmaster -c sort_mem=1234 $ postmaster --sort-mem=1234
Either form overrides whatever setting might exist for sort_mem in postgresql.conf. Notice that underscores in parameter names can be written as either underscore or dash on the command line.
Tip: Except for short-term experiments, it's probably better practice to edit the setting in postgresql.conf than to rely on a command-line switch to set a parameter.