POSTMASTER(UNIX)

NAME
postmaster - run the Postgres postmaster

SYNOPSIS
postmaster [-B n_buffers] [-D data_dir] [-N n_backends]
[-S ] [-a system] [-b backend_pathname] [-d [debug_level]] [-i ] [-n ] [-o backend_options] [-p port] [-s ]

DESCRIPTION
The postmaster manages the communication between frontend and backend processes, as well as allocating the shared buffer pool and SysV semaphores (on machines without a test-and-set instruction). The postmaster does not itself interact with the user and should be started as a background process.

Only one postmaster should be running at a time in a given Postgres installation." Here, an installation means a database directory and postmaster port number. You can run more than one postmaster on a machine only if each one has a separate directory and port number.

The postmaster understands the following command-line options:

-B n_buffers
    n_buffers is the number of shared-memory buffers for the postmaster to allocate and manage for the backend server processes that it starts. This value defaults to 64 buffers, where each buffer is 8k bytes (or whatever BLCKSZ is set to in config.h).

-D data_dir
    Specifies the directory to use as the root of the tree of database directories. If -D is not given, the default data directory name is the value of the environment variable PGDATA. If PGDATA is not set, then the directory used is $POSTGRESHOME /data. If neither environment variable is set and this command-line option is not specified, the default directory that was set at compile-time is used.

-N n_backends
    n_backends is the maximum number of backend server processes that this postmaster is allowed to start. In the stock configuration, this value defaults to 32, and can be set as high as 1024 if your system will support that many processes. Both the default and upper limit values can be altered when building Postgres (see src/include/config.h).

-S  Specifies that the postmaster process should start up in silent mode. That is, it will disassociate from the user's (controlling) tty and start its own process group. This should not be used in combination with debugging options because any messages printed to standard output and standard error are discarded.

-a system
    Specifies whether or not to use the authentication system system for frontend applications to use in connecting to the postmaster process. Specify system to enable a system, or nosystem to disable a system. For example, to permit users to use Kerberos authentication, use -a kerberos; to deny any unauthenticated connections, use -a nounauth . This option no longer has any effect! The pg_hba.conf configuration file now controls authentication.

-b backend_pathname
    backend_pathname is the full pathname of the Postgres backend server executable file that the postmaster will invoke when it receives a connection from a frontend application. If this option is not used, then the postmaster tries to find this executable file in the directory in which its own executable is located (this is done by looking at the pathname under which the postmaster was invoked. If no pathname was specified, then the PATH environment variable is searched for an executable named “postgres”). This option is also obsolete, since the postmaster and backend are now a single executable --- the postmaster forks a backend but does not do an exec.

-d [debug_level]
    The optional argument debug_level determines the amount of debugging output the backend servers will produce. If debug_level is one, the postmaster will trace all connection traffic, and nothing else. For levels two and higher, debugging is turned on in the backend process and the postmaster displays more information, including the backend environment and process traffic. Note that if no file is specified for backend servers to send their debugging output then this output will appear on the controlling tty of their parent postmaster.

-i  This enables TCP/IP or Internet domain socket communication. Without this option, only local Unix domain socket communication is possible.

-n, -s
    The -s and -n options control the behavior of the postmaster when a backend dies abnormally. 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.

    If the -s option is supplied, then the 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 the -n option is supplied, then the postmaster does not reinitialize shared data structures. A knowledgable system programmer can then use the shmemdoc program to examine shared memory and semaphore state.

-o backend_options
    The postgres(1) options specified in backend_options are passed to all backend server processes started by this postmaster. If the option string contains any spaces, the entire string must be quoted.

-p port
    Specifies the TCP/IP port or local Unix domain socket file extension on which the postmaster is to listen for connections from frontend applications. Defaults to the value of the PGPORT environment variable, or if PGPORT is not set, then the default value established when Postgres was compiled (normally 5432). If you specify a port other than the default port then all frontend application users must specify the same port (using command-line options or PGPORT ) when starting any libpq application, including psql.

WARNINGS
If at all possible, do not use SIGKILL when killing the postmaster. SIGHUP, SIGINT, or SIGTERM (the default signal for kill(1)) should be used instead. Hence, avoid
kill -KILL
or its alternative form
kill -9
as this will prevent the postmaster from freeing the system resources (e.g., shared memory and semaphores) that it holds before dying. This prevents you from having to deal with the problem with shmat(2) described below.

EXAMPLES
# start postmaster using default values
nohup postmaster >logfile 2>&1 &
This command will start up postmaster on the default port (5432) and will search $PATH to find an executable file called “postgres”. This is the simplest and most common way to start the postmaster.
# start with specific port and executable name
nohup postmaster -p 1234 -b /usr/postgres/bin/postgres &
This command will start up a postmaster communicating through the port 1234, and will attempt to use the backend located at “/usr/postgres/bin/postgres”. In order to connect to this postmaster using psql, you would need to either specify -p 1234 on the psql command-line or set the environment variable PGPORT to 1234.

SEE ALSO
ipcs(1), ipcrm(1), ipcclean(1), psql(1), postgres(1),

DIAGNOSTICS

FindBackend: could not find a backend to execute...
    If you see this message, you do not have the postgres executable in your path. Add the directory in which postgres resides to your path.

semget: No space left on device
    If you see this message, you should run the ipcclean command. After doing this, try starting the 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 postmasters 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. (You may be able to postpone reconfiguring your kernel by reducing -B to reduce Postgres' shared memory consumption, or by reducing -N to reduce Postgres' semaphore consumption.)

StreamServerPort: cannot bind to port
    If you see this message, you should be certain that there is no other postmaster process already running. The easiest way to determine this is by using the command
    ps -ax | grep postmaster
    on BSD-based systems
    ps -e | grep postmast
    (the equivalent syntax is on System V-like or POSIX-compliant systems such as HP-UX). 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.

IpcMemoryAttach: shmat() failed: Permission denied
    A likely explanation is that another user attempted to start a postmaster process on the same port which acquired shared resources and then died. Since Postgres shared memory keys are based on the port number assigned to the postmaster, such conflicts are likely if there is more than one installation on a single host. If there are no other postmaster processes currently running (see above), run ipcclean and try again. If other postmasters are running, you will have to find the owners of those processes to coordinate the assignment of port numbers and/or removal of unused shared memory segments.