table of contents
| PG_RECEIVEWAL(1) | PostgreSQL 13.17 Documentation | PG_RECEIVEWAL(1) |
NAME¶
pg_receivewal - stream write-ahead logs from a PostgreSQL server
SYNOPSIS¶
pg_receivewal [option...]
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 user having REPLICATION permissions (see Section 21.2) or a superuser, 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:
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.
In the absence of fatal errors, pg_receivewal will run until terminated by the SIGINT signal (Control+C).
OPTIONS¶
-D directory
--directory=directory
This parameter is required.
-E lsn
--endpos=lsn
If there is a record with LSN exactly equal to lsn, the record will be processed.
--if-not-exists
-n
--no-loop
--no-sync
This option is incompatible with --synchronous.
-s interval
--status-interval=interval
-S slotname
--slot=slotname
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
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
-Z level
--compress=level
The following command-line options control the database connection parameters.
-d connstr
--dbname=connstr
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
-p port
--port=port
-U username
--username=username
-w
--no-password
-W
--password
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
--drop-slot
Other options are also available:
-V
--version
-?
--help
EXIT STATUS¶
pg_receivewal will exit with status 0 when terminated by the SIGINT signal. (That is the normal way to end it. Hence it is not an error.) For fatal errors or other signals, the exit status will be nonzero.
ENVIRONMENT¶
This utility, like most other PostgreSQL utilities, uses the environment variables supported by libpq (see Section 33.14).
The environment variable PG_COLOR specifies whether to use color in diagnostic messages. Possible values are always, auto and never.
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.
pg_receivewal will preserve group permissions on the received WAL files if group permissions are enabled on the source cluster.
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
SEE ALSO¶
| 2024 | PostgreSQL 13.17 |