PostgreSQL: The World's Most Advanced Open Source Relational Database

pg_receivewal
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Description

pg_receivewal is used to stream the write-ahead log from a running PostgreSQL cluster. The write-ahead log is streamed using the streaming replication protocol, and is written to a local directory of files. This directory can be used as the archive location for doing a restore using point-in-time recovery (see Section 25.3 ).

pg_receivewal streams the write-ahead log in real time as it's being generated on the server, and does not wait for segments to complete like archive_command does. For this reason, it is not necessary to set archive_timeout when using pg_receivewal .

Unlike the WAL receiver of a PostgreSQL standby server, pg_receivewal by default flushes WAL data only when a WAL file is closed. The option --synchronous must be specified to flush WAL data in real time. Since pg_receivewal does not apply WAL, you should not allow it to become a synchronous standby when synchronous_commit equals remote_apply . If it does, it will appear to be a standby that never catches up, and will cause transaction commits to block. To avoid this, you should either configure an appropriate value for synchronous_standby_names , or specify application_name for pg_receivewal that does not match it, or change the value of synchronous_commit to something other than remote_apply .

The write-ahead log is streamed over a regular PostgreSQL connection and uses the replication protocol. The connection must be made with a superuser or a user having REPLICATION permissions (see Section 21.2 ), and pg_hba.conf must permit the replication connection. The server must also be configured with max_wal_senders set high enough to leave at least one session available for the stream.

The starting point of the write-ahead log streaming is calculated when pg_receivewal starts:

First, scan the directory where the WAL segment files are written and find the newest completed segment file, using as the starting point the beginning of the next WAL segment file.
If a starting point cannot be calculated with the previous method, the latest WAL flush location is used as reported by the server from an IDENTIFY_SYSTEM command.

If the connection is lost, or if it cannot be initially established, with a non-fatal error, pg_receivewal will retry the connection indefinitely, and reestablish streaming as soon as possible. To avoid this behavior, use the -n parameter.

Options

-D directory --directory= directory

Directory to write the output to.

This parameter is required.

--if-not-exists

Do not error out when --create-slot is specified and a slot with the specified name already exists.

-n --no-loop

Don't loop on connection errors. Instead, exit right away with an error.

-s interval --status-interval= interval

Specifies the number of seconds between status packets sent back to the server. This allows for easier monitoring of the progress from server. A value of zero disables the periodic status updates completely, although an update will still be sent when requested by the server, to avoid timeout disconnect. The default value is 10 seconds.

-S slotname --slot= slotname

Require pg_receivewal to use an existing replication slot (see Section 26.2.6 ). When this option is used, pg_receivewal will report a flush position to the server, indicating when each segment has been synchronized to disk so that the server can remove that segment if it is not otherwise needed.

When the replication client of pg_receivewal is configured on the server as a synchronous standby, then using a replication slot will report the flush position to the server, but only when a WAL file is closed. Therefore, that configuration will cause transactions on the primary to wait for a long time and effectively not work satisfactorily. The option --synchronous (see below) must be specified in addition to make this work correctly.

--synchronous

Flush the WAL data to disk immediately after it has been received. Also send a status packet back to the server immediately after flushing, regardless of --status-interval .

This option should be specified if the replication client of pg_receivewal is configured on the server as a synchronous standby, to ensure that timely feedback is sent to the server.

-v --verbose

Enables verbose mode.

-Z level --compress= level

Enables gzip compression of write-ahead logs, and specifies the compression level (0 through 9, 0 being no compression and 9 being best compression). The suffix .gz will automatically be added to all filenames.

The following command-line options control the database connection parameters.

-d connstr --dbname= connstr

Specifies parameters used to connect to the server, as a connection string ; these will override any conflicting command line options.

The option is called --dbname for consistency with other client applications, but because pg_receivewal doesn't connect to any particular database in the cluster, database name in the connection string will be ignored.

-h host --host= host

Specifies the host name of the machine on which the server is running. If the value begins with a slash, it is used as the directory for the Unix domain socket. The default is taken from the PGHOST environment variable, if set, else a Unix domain socket connection is attempted.

-p port --port= port

Specifies the TCP port or local Unix domain socket file extension on which the server is listening for connections. Defaults to the PGPORT environment variable, if set, or a compiled-in default.

-U username --username= username

User name to connect as.

-w --no-password

Never issue a password prompt. If the server requires password authentication and a password is not available by other means such as a .pgpass file, the connection attempt will fail. This option can be useful in batch jobs and scripts where no user is present to enter a password.

-W --password

Force pg_receivewal to prompt for a password before connecting to a database.

This option is never essential, since pg_receivewal will automatically prompt for a password if the server demands password authentication. However, pg_receivewal will waste a connection attempt finding out that the server wants a password. In some cases it is worth typing -W to avoid the extra connection attempt.

pg_receivewal can perform one of the two following actions in order to control physical replication slots:

--create-slot: Create a new physical replication slot with the name specified in --slot , then exit.
--drop-slot: Drop the replication slot with the name specified in --slot , then exit.

Other options are also available:

-V --version: Print the pg_receivewal version and exit.
-? --help: Show help about pg_receivewal command line arguments, and exit.

Environment

This utility, like most other PostgreSQL utilities, uses the environment variables supported by libpq (see Section 33.14 ).

Notes

When using pg_receivewal instead of archive_command as the main WAL backup method, it is strongly recommended to use replication slots. Otherwise, the server is free to recycle or remove write-ahead log files before they are backed up, because it does not have any information, either from archive_command or the replication slots, about how far the WAL stream has been archived. Note, however, that a replication slot will fill up the server's disk space if the receiver does not keep up with fetching the WAL data.

Examples

To stream the write-ahead log from the server at mydbserver and store it in the local directory /usr/local/pgsql/archive :

$ pg_receivewal -h mydbserver -D /usr/local/pgsql/archive

pg_receivewal