pg_receivewal — stream write-ahead logs from a PostgreSQL server
pg_receivewal
[option
...]
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.
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.
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.
-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. See Section 33.1.1 for more information.
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.
This utility, like most other PostgreSQL utilities, uses the environment variables supported by libpq (see Section 33.14).
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.
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