PostgreSQL : Documentation: 9.6: pg_basebackup : Postgres Professional

pg_basebackup
Prev	Up	PostgreSQL Client Applications	Home	Next

pg_basebackup

pg_basebackup — take a base backup of a PostgreSQL cluster

Synopsis

pg_basebackup [option...]

Description

pg_basebackup is used to take base backups of a running PostgreSQL database cluster. These are taken without affecting other clients to the database, and can be used both for point-in-time recovery (see Section 25.3) and as the starting point for a log shipping or streaming replication standby servers (see Section 26.2).

pg_basebackup makes a binary copy of the database cluster files, while making sure the system is put in and out of backup mode automatically. Backups are always taken of the entire database cluster; it is not possible to back up individual databases or database objects. For individual database backups, a tool such as pg_dump must be used.

The backup is made 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 explicitly 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 backup.

There can be multiple pg_basebackups running at the same time, but it is better from a performance point of view to take only one backup, and copy the result.

pg_basebackup can make a base backup from not only the master but also the standby. To take a backup from the standby, set up the standby so that it can accept replication connections (that is, set max_wal_senders and hot_standby, and configure host-based authentication). You will also need to enable full_page_writes on the master.

Note that there are some limitations in an online backup from the standby:

The backup history file is not created in the database cluster backed up.
There is no guarantee that all WAL files required for the backup are archived at the end of backup. If you are planning to use the backup for an archive recovery and want to ensure that all required files are available at that moment, you need to include them into the backup by using the -x option.
If the standby is promoted to the master during online backup, the backup fails.
All WAL records required for the backup must contain sufficient full-page writes, which requires you to enable full_page_writes on the master and not to use a tool like pg_compresslog as archive_command to remove full-page writes from WAL files.

Options

The following command-line options control the location and format of the output.

-D directory --pgdata=directory

Directory to write the output to. pg_basebackup will create the directory and any parent directories if necessary. The directory may already exist, but it is an error if the directory already exists and is not empty.

When the backup is in tar mode, and the directory is specified as - (dash), the tar file will be written to stdout.

This option is required.

-F format --format=format

Selects the format for the output. format can be one of the following:

p plain

Write the output as plain files, with the same layout as the current data directory and tablespaces. When the cluster has no additional tablespaces, the whole database will be placed in the target directory. If the cluster contains additional tablespaces, the main data directory will be placed in the target directory, but all other tablespaces will be placed in the same absolute path as they have on the source server. (See --tablespace-mapping to change that.)

This is the default format.

t tar

Write the output as tar files in the target directory. The main data directory will be written to a file named base.tar, and all other tablespaces will be named after the tablespace OID.

If the value - (dash) is specified as target directory, the tar contents will be written to standard output, suitable for piping to for example gzip. This is only possible if the cluster has no additional tablespaces.

-r rate --max-rate=rate

The maximum transfer rate of data transferred from the server. Values are in kilobytes per second. Use a suffix of M to indicate megabytes per second. A suffix of k is also accepted, and has no effect. Valid values are between 32 kilobytes per second and 1024 megabytes per second.

The purpose is to limit the impact of pg_basebackup on the running server.

This option always affects transfer of the data directory. Transfer of WAL files is only affected if the collection method is fetch.

-R --write-recovery-conf

Write a minimal recovery.conf in the output directory (or into the base archive file when using tar format) to ease setting up a standby server. The recovery.conf file will record the connection settings and, if specified, the replication slot that pg_basebackup is using, so that the streaming replication will use the same settings later on.

-S slotname --slot=slotname

This option can only be used together with -X stream. It causes the WAL streaming to use the specified replication slot. If the base backup is intended to be used as a streaming replication standby using replication slots, it should then use the same replication slot name in recovery.conf. That way, it is ensured that the server does not remove any necessary WAL data in the time between the end of the base backup and the start of streaming replication.

-T olddir=newdir --tablespace-mapping=olddir=newdir

Relocate the tablespace in directory olddir to newdir during the backup. To be effective, olddir must exactly match the path specification of the tablespace as it is currently defined. (But it is not an error if there is no tablespace in olddir contained in the backup.) Both olddir and newdir must be absolute paths. If a path happens to contain a = sign, escape it with a backslash. This option can be specified multiple times for multiple tablespaces. See examples below.

If a tablespace is relocated in this way, the symbolic links inside the main data directory are updated to point to the new location. So the new data directory is ready to be used for a new server instance with all tablespaces in the updated locations.

Currently, this option only works with plain output format; it is ignored if tar format is selected.

--xlogdir=xlogdir

Specifies the location for the transaction log directory. xlogdir must be an absolute path. The transaction log directory can only be specified when the backup is in plain mode.

-x --xlog

Using this option is equivalent of using -X with method fetch.

-X method --xlog-method=method

Includes the required transaction log files (WAL files) in the backup. This will include all transaction logs generated during the backup. If this option is specified, it is possible to start a postmaster directly in the extracted directory without the need to consult the log archive, thus making this a completely standalone backup.

The following methods for collecting the transaction logs are supported:

f fetch: The transaction log files are collected at the end of the backup. Therefore, it is necessary for the wal_keep_segments parameter to be set high enough that the log is not removed before the end of the backup. If the log has been rotated when it's time to transfer it, the backup will fail and be unusable.
s stream: Stream the transaction log while the backup is created. This will open a second connection to the server and start streaming the transaction log in parallel while running the backup. Therefore, it will use up two connections configured by the max_wal_senders parameter. As long as the client can keep up with transaction log received, using this mode requires no extra transaction logs to be saved on the master.

-z --gzip

Enables gzip compression of tar file output, with the default compression level. Compression is only available when using the tar format.

-Z level --compress=level

Enables gzip compression of tar file output, and specifies the compression level (0 through 9, 0 being no compression and 9 being best compression). Compression is only available when using the tar format.

The following command-line options control the generation of the backup and the running of the program.

-c fast|spread --checkpoint=fast|spread

Sets checkpoint mode to fast (immediate) or spread (default) (see Section 25.3.3).

-l label --label=label

Sets the label for the backup. If none is specified, a default value of “pg_basebackup base backup” will be used.

-P --progress

Enables progress reporting. Turning this on will deliver an approximate progress report during the backup. Since the database may change during the backup, this is only an approximation and may not end at exactly 100%. In particular, when WAL log is included in the backup, the total amount of data cannot be estimated in advance, and in this case the estimated target size will increase once it passes the total estimate without WAL.

When this is enabled, the backup will start by enumerating the size of the entire database, and then go back and send the actual contents. This may make the backup take slightly longer, and in particular it will take longer before the first data is sent.

-v --verbose

Enables verbose mode. Will output some extra steps during startup and shutdown, as well as show the exact file name that is currently being processed if progress reporting is also enabled.

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_basebackup 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.

-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.

-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_basebackup to prompt for a password before connecting to a database.

This option is never essential, since pg_basebackup will automatically prompt for a password if the server demands password authentication. However, pg_basebackup 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.

Other options are also available:

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

Environment

This utility, like most other PostgreSQL utilities, uses the environment variables supported by libpq (see Section 32.14).

Notes

At the beginning of the backup, a checkpoint needs to be written on the server the backup is taken from. Especially if the option --checkpoint=fast is not used, this can take some time during which pg_basebackup will be appear to be idle.

The backup will include all files in the data directory and tablespaces, including the configuration files and any additional files placed in the directory by third parties. But only regular files and directories are copied. Symbolic links (other than those used for tablespaces) and special device files are skipped. (See Section 51.3 for the precise details.)

Tablespaces will in plain format by default be backed up to the same path they have on the server, unless the option --tablespace-mapping is used. Without this option, running a plain format base backup on the same host as the server will not work if tablespaces are in use, because the backup would have to be written to the same directory locations as the original tablespaces.

When tar format mode is used, it is the user's responsibility to unpack each tar file before starting the PostgreSQL server. If there are additional tablespaces, the tar files for them need to be unpacked in the correct locations. In this case the symbolic links for those tablespaces will be created by the server according to the contents of the tablespace_map file that is included in the base.tar file.

pg_basebackup works with servers of the same or an older major version, down to 9.1. However, WAL streaming mode (-X stream) only works with server version 9.3 and later, and tar format mode (--format=tar) of the current version only works with server version 9.5 or later.

Examples

To create a base backup of the server at mydbserver and store it in the local directory /usr/local/pgsql/data:

$ pg_basebackup -h mydbserver -D /usr/local/pgsql/data

To create a backup of the local server with one compressed tar file for each tablespace, and store it in the directory backup, showing a progress report while running:

$ pg_basebackup -D backup -Ft -z -P

To create a backup of a single-tablespace local database and compress this with bzip2:

$ pg_basebackup -D - -Ft | bzip2 > backup.tar.bz2

(This command will fail if there are multiple tablespaces in the database.)

To create a backup of a local database where the tablespace in /opt/ts is relocated to ./backup/ts:

$ pg_basebackup -D backup/data -T /opt/ts=$(pwd)/backup/ts