Starting and Stopping Greenplum Database

In a Greenplum Database DBMS, the database server instances (the master and all segments) are started or stopped across all of the hosts in the system in such a way that they can work together as a unified DBMS.

Because a Greenplum Database system is distributed across many machines, the process for starting and stopping a Greenplum Database system is different than the process for starting and stopping a regular PostgreSQL DBMS.

Use the gpstart and gpstop utilities to start and stop Greenplum Database, respectively. These utilities are located in the $GPHOME/bin directory on your Greenplum Database master host.

Important: Do not issue a kill command to end any Postgres process. Instead, use the database command pg_cancel_backend().

Issuing a kill -9 or kill -11 might introduce database corruption. If Pivotal Greenplum Database corruption occurs, Pivotal will make best efforts to restore a cluster. A root cause analysis cannot be performed.

For information about gpstart and gpstop, see the Greenplum Database Utility Guide.

Starting Greenplum Database

Start an initialized Greenplum Database system by running the gpstart utility on the master instance.

Use the gpstart utility to start a Greenplum Database system that has already been initialized by the gpinitsystem utility, but has been stopped by the gpstop utility. The gpstart utility starts Greenplum Database by starting all the Postgres database instances on the Greenplum Database cluster. gpstart orchestrates this process and performs the process in parallel.

Run gpstart on the master host to start Greenplum Database:

$ gpstart

Restarting Greenplum Database

Stop the Greenplum Database system and then restart it.

The gpstop utility with the -r option can stop and then restart Greenplum Database after the shutdown completes.

To restart Greenplum Database, enter the following command on the master host:

$ gpstop -r

Reloading Configuration File Changes Only

Reload changes to Greenplum Database configuration files without interrupting the system.

The gpstop utility can reload changes to the pg_hba.conf configuration file and to runtime parameters in the master postgresql.conf file and pg_hba.conf file without service interruption. Active sessions pick up changes when they reconnect to the database. Many server configuration parameters require a full system restart (gpstop -r) to activate. For information about server configuration parameters, see the Greenplum Database Reference Guide.

Reload configuration file changes without shutting down the system using the gpstop utility:

$ gpstop -u

Starting the Master in Maintenance Mode

Start only the master to perform maintenance or administrative tasks without affecting data on the segments.

Maintenance mode should only be used with direction from Pivotal Technical Support. For example, you could connect to a database only on the master instance in maintenance mode and edit system catalog settings. For more information about system catalog tables, see the Greenplum Database Reference Guide.

Run gpstart using the -m option:
```
$ gpstart -m
```
Connect to the master in maintenance mode to do catalog maintenance. For example:
```
$ PGOPTIONS='-c gp_session_role=utility' psql template1
```
After completing your administrative tasks, stop the master in utility mode. Then, restart it in production mode.
```
$ gpstop -mr
```
Warning:
Incorrect use of maintenance mode connections can result in an inconsistent system state. Only Technical Support should perform this operation.

Stopping Greenplum Database

The gpstop utility stops or restarts your Greenplum Database system and always runs on the master host. When activated, gpstop stops all postgres processes in the system, including the master and all segment instances. The gpstop utility uses a default of up to 64 parallel worker threads to bring down the Postgres instances that make up the Greenplum Database cluster. The system waits for any active transactions to finish before shutting down. To stop Greenplum Database immediately, use fast mode.

To stop Greenplum Database:
```
$ gpstop
```
To stop Greenplum Database in fast mode:
```
$ gpstop -M fast
```

Stopping Client Processes

Greenplum Database launches a new backend process for each client connection. A Greenplum Database user with SUPERUSER privileges can cancel and terminate these client backend processes.

Canceling a backend process with the pg_cancel_backend() function ends a specific queued or active client query. Terminating a backend process with the pg_terminate_backend() function terminates a client connection to a database. The signatures of these functions are:

pg_cancel_backend( int4 pid )
pg_terminate_backend( int4 pid )

pg_cancel_backend() and pg_terminate_backend() return true if successful, and false otherwise.

To cancel or terminate a backend process, you must first identify the process ID of the backend. You can obtain the process ID from the procpid column of the pg_stat_activity view. For example, to view the process information associated with all running and queued queries:

=# SELECT usename, procpid, waiting, current_query, datname
     FROM pg_stat_activity;

Sample partial query output:

 usename |  procpid | waiting |     current_query     | datname
---------+----------+---------+-----------------------+---------
  sammy  |   31861  |    f    | <IDLE> in transaction | testdb
  billy  |   31905  |    t    | SELECT * FROM topten; | testdb

Use the output to identify the process id (procpid) of the query or client connection.

For example, to cancel the pending query identified in the sample output above:

=# SELECT pg_cancel_backend( 31905 );