lt_receivewal — stream write-ahead logs from a LightDB server
lt_receivewal [option...]
lt_receivewal is used to stream the write-ahead log from a running LightDB 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 23.3).
lt_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 lt_receivewal.
Unlike the WAL receiver of a LightDB standby server, lt_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 lt_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
lt_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
LightDB connection and uses the replication
protocol. The connection must be made with a user having
REPLICATION permissions (see
Section 19.2) or a superuser, and
lt_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 lt_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, lt_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, lt_receivewal will run until terminated by the SIGINT signal (Control+C).
-D directory--directory=directoryDirectory to write the output to.
This parameter is required.
-E lsn--endpos=lsnAutomatically stop replication and exit with normal exit status 0 when receiving reaches the specified LSN.
If there is a record with LSN exactly equal to lsn,
the record will be processed.
--if-not-exists
Do not error out when --create-slot is specified
and a slot with the specified name already exists.
-n--no-loopDon't loop on connection errors. Instead, exit right away with an error.
--no-sync
This option causes lt_receivewal to not force WAL
data to be flushed to disk. This is faster, but means that a
subsequent operating system crash can leave the WAL segments corrupt.
Generally, this option is useful for testing but should not be used
when doing WAL archiving on a production deployment.
This option is incompatible with --synchronous.
-s interval--status-interval=intervalSpecifies 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=slotnameRequire lt_receivewal to use an existing replication slot (see Section 24.2.6). When this option is used, lt_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 lt_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 lt_receivewal is configured on the server as a synchronous standby, to ensure that timely feedback is sent to the server.
-v--verboseEnables 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=connstrSpecifies 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 lt_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 LTHOST 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 LTPORT environment variable, if
set, or a compiled-in default.
-U username--username=usernameUser 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--passwordForce lt_receivewal to prompt for a password before connecting to a database.
This option is never essential, since
lt_receivewal will automatically prompt
for a password if the server demands password authentication.
However, lt_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.
lt_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--versionPrint the lt_receivewal version and exit.
-?--helpShow help about lt_receivewal command line arguments, and exit.
lt_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.
This utility, like most other LightDB utilities, uses the environment variables supported by libpq (see Section 32.14).
The environment variable LT_COLOR specifies whether to use
color in diagnostic messages. Possible values are
always, auto and
never.
When using lt_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.
lt_receivewal will preserve group permissions on the received WAL files if group permissions are enabled on the source cluster.
To stream the write-ahead log from the server at
mydbserver and store it in the local directory
/usr/local/pgsql/archive:
$lt_receivewal -h mydbserver -D /usr/local/pgsql/archive