pg_basebackup(1) - phpMan

PG_BASEBACKUP(1) PostgreSQL 14.20 Documentation PG_BASEBACKUP(1)

NAME
pg_basebackup - take a base backup of a PostgreSQL cluster

SYNOPSIS
pg_basebackup [option...]

DESCRIPTION
pg_basebackup is used to take a base backup of a running PostgreSQL database cluster. The
backup is taken without affecting other clients of the database, and can be used both for
point-in-time recovery (see Section 26.3) and as the starting point for a log-shipping or
streaming-replication standby server (see Section 27.2).

pg_basebackup makes an exact copy of the database cluster's files, while making sure the
server is put into 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 selective backups, another tool such as pg_dump(1) must be used.

The backup is made over a regular PostgreSQL connection that uses the replication
protocol. The connection must be made with a user ID that has REPLICATION permissions (see
Section 22.2) or is 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 provide at
least one walsender for the backup plus one for WAL streaming (if used).

There can be multiple pg_basebackups running at the same time, but it is usually 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 a primary server but also a standby. To
take a backup from a standby, set up the standby so that it can accept replication
connections (that is, set max_wal_senders and hot_standby, and configure its pg_hba.conf
appropriately). You will also need to enable full_page_writes on the primary.

Note that there are some limitations in taking a backup from a standby:

o The backup history file is not created in the database cluster backed up.

o pg_basebackup cannot force the standby to switch to a new WAL file at the end of
backup. When you are using -X none, if write activity on the primary is low,
pg_basebackup may need to wait a long time for the last WAL file required for the
backup to be switched and archived. In this case, it may be useful to run
pg_switch_wal on the primary in order to trigger an immediate WAL file switch.

o If the standby is promoted to be primary during backup, the backup fails.

o All WAL records required for the backup must contain sufficient full-page writes,
which requires you to enable full_page_writes on the primary and not to use a tool
like pg_compresslog as archive_command to remove full-page writes from WAL files.

Whenever pg_basebackup is taking a base backup, the server's pg_stat_progress_basebackup
view will report the progress of the backup. See Section 28.4.5 for details.

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

-D directory
--pgdata=directory
Sets the target directory to write the output to. pg_basebackup will create this
directory (and any missing parent directories) if it does not exist. If it already
exists, it must be empty.

When the backup is in tar format, the target directory may be specified as - (dash),
causing the tar file to 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 source server's 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's
contents will be written to a file named base.tar, and each other tablespace will
be written to a separate tar file named after that tablespace's OID.

If the target directory is specified as - (dash), the tar contents will be written
to standard output, suitable for piping to (for example) gzip. This is only
allowed if the cluster has no additional tablespaces and WAL streaming is not
used.

-R
--write-recovery-conf
Creates a standby.signal

file and appends connection settings to the postgresql.auto.conf file in the target
directory (or within the base archive file when using tar format). This eases setting
up a standby server using the results of the backup.

The postgresql.auto.conf file will record the connection settings and, if specified,
the replication slot that pg_basebackup is using, so that streaming replication will
use the same settings later on.

-T olddir=newdir
--tablespace-mapping=olddir=newdir
Relocates 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
defined on the source server. (But it is not an error if there is no tablespace in
olddir on the source server.) Meanwhile newdir is a directory in the receiving host's
filesystem. As with the main target directory, newdir need not exist already, but if
it does exist it must be empty. Both olddir and newdir must be absolute paths. If
either path needs to contain an equal sign (=), precede that with a backslash. This
option can be specified multiple times for multiple tablespaces.

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.

--waldir=waldir
Sets the directory to write WAL (write-ahead log) files to. By default WAL files will
be placed in the pg_wal subdirectory of the target directory, but this option can be
used to place them elsewhere. waldir must be an absolute path. As with the main
target directory, waldir need not exist already, but if it does exist it must be
empty. This option can only be specified when the backup is in plain format.

-X method
--wal-method=method
Includes the required WAL (write-ahead log) files in the backup. This will include all
write-ahead logs generated during the backup. Unless the method none is specified, it
is possible to start a postmaster in the target directory without the need to consult
the log archive, thus making the output a completely standalone backup.

The following methods for collecting the write-ahead logs are supported:

n
none
Don't include write-ahead logs in the backup.

f
fetch
The write-ahead log files are collected at the end of the backup. Therefore, it is
necessary for the source server's wal_keep_size parameter to be set high enough
that the required log data is not removed before the end of the backup. If the
required log data has been recycled before it's time to transfer it, the backup
will fail and be unusable.

When tar format is used, the write-ahead log files will be included in the
base.tar file.

s
stream
Stream write-ahead log data while the backup is being taken. This method will open
a second connection to the server and start streaming the write-ahead log in
parallel while running the backup. Therefore, it will require two replication
connections not just one. As long as the client can keep up with the write-ahead
log data, using this method requires no extra write-ahead logs to be saved on the
source server.

When tar format is used, the write-ahead log files will be written to a separate
file named pg_wal.tar (if the server is a version earlier than 10, the file will
be named pg_xlog.tar).

This value is the default.

-z
--gzip
Enables gzip compression of tar file output, with the default compression level.
Compression is only available when using the tar format, and the suffix .gz will
automatically be added to all tar filenames.

-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, and the suffix .gz will automatically be added to
all tar filenames.

The following command-line options control the generation of the backup and the invocation
of the program:

-c fast|spread
--checkpoint=fast|spread
Sets checkpoint mode to fast (immediate) or spread (the default) (see Section 26.3.3).

-C
--create-slot
Specifies that the replication slot named by the --slot option should be created
before starting the backup. An error is raised if the slot already exists.

-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.

-n
--no-clean
By default, when pg_basebackup aborts with an error, it removes any directories it
might have created before discovering that it cannot finish the job (for example, the
target directory and write-ahead log directory). This option inhibits tidying-up and
is thus useful for debugging.

Note that tablespace directories are not cleaned up either way.

-N
--no-sync
By default, pg_basebackup will wait for all files to be written safely to disk. This
option causes pg_basebackup to return without waiting, which is faster, but means that
a subsequent operating system crash can leave the base backup corrupt. Generally, this
option is useful for testing but should not be used when creating a production
installation.

-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.

-r rate
--max-rate=rate
Sets the maximum transfer rate at which data is collected from the source server. This
can be useful to limit the impact of pg_basebackup on 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.

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

-S slotname
--slot=slotname
This option can only be used together with -X stream. It causes WAL streaming to use
the specified replication slot. If the base backup is intended to be used as a
streaming-replication standby using a replication slot, the standby should then use
the same replication slot name as primary_slot_name. This ensures that the primary
server does not remove any necessary WAL data in the time between the end of the base
backup and the start of streaming replication on the new standby.

The specified replication slot has to exist unless the option -C is also used.

If this option is not specified and the server supports temporary replication slots
(version 10 and later), then a temporary replication slot is automatically used for
WAL streaming.

-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.

--manifest-checksums=algorithm
Specifies the checksum algorithm that should be applied to each file included in the
backup manifest. Currently, the available algorithms are NONE, CRC32C, SHA224, SHA256,
SHA384, and SHA512. The default is CRC32C.

If NONE is selected, the backup manifest will not contain any checksums. Otherwise, it
will contain a checksum of each file in the backup using the specified algorithm. In
addition, the manifest will always contain a SHA256 checksum of its own contents. The
SHA algorithms are significantly more CPU-intensive than CRC32C, so selecting one of
them may increase the time required to complete the backup.

Using a SHA hash function provides a cryptographically secure digest of each file for
users who wish to verify that the backup has not been tampered with, while the CRC32C
algorithm provides a checksum that is much faster to calculate; it is good at catching
errors due to accidental changes but is not resistant to malicious modifications. Note
that, to be useful against an adversary who has access to the backup, the backup
manifest would need to be stored securely elsewhere or otherwise verified not to have
been modified since the backup was taken.

pg_verifybackup(1) can be used to check the integrity of a backup against the backup
manifest.

--manifest-force-encode
Forces all filenames in the backup manifest to be hex-encoded. If this option is not
specified, only non-UTF8 filenames are hex-encoded. This option is mostly intended to
test that tools which read a backup manifest file properly handle this case.

--no-estimate-size
Prevents the server from estimating the total amount of backup data that will be
streamed, resulting in the backup_total column in the pg_stat_progress_basebackup view
always being NULL.

Without this option, 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.
This option is useful to avoid such estimation time if it's too long.

This option is not allowed when using --progress.

--no-manifest
Disables generation of a backup manifest. If this option is not specified, the server
will generate and send a backup manifest which can be verified using
pg_verifybackup(1). The manifest is a list of every file present in the backup with
the exception of any WAL files that may be included. It also stores the size, last
modification time, and an optional checksum for each file.

--no-slot
Prevents the creation of a temporary replication slot for the backup.

By default, if log streaming is selected but no slot name is given with the -S option,
then a temporary replication slot is created (if supported by the source server).

The main purpose of this option is to allow taking a base backup when the server has
no free replication slots. Using a replication slot is almost always preferred,
because it prevents needed WAL from being removed by the server during the backup.

--no-verify-checksums
Disables verification of checksums, if they are enabled on the server the base backup
is taken from.

By default, checksums are verified and checksum failures will result in a non-zero
exit status. However, the base backup will not be removed in such a case, as if the
--no-clean option had been used. Checksum verification failures will also be reported
in the pg_stat_database view.

The following command-line options control the connection to the source server:

-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, any
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 a 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 source server.
Smaller values allow more accurate monitoring of backup progress from the server. A
value of zero disables periodic status updates completely, although an update will
still be sent when requested by the server, to avoid timeout-based disconnects. The
default value is 10 seconds.

-U username
--username=username
Specifies the user name to connect as.

-w
--no-password
Prevents issuing 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
Forces pg_basebackup to prompt for a password before connecting to the source server.

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
Prints the pg_basebackup version and exits.

-?
--help
Shows help about pg_basebackup command line arguments, and exits.

ENVIRONMENT
This utility, like most other PostgreSQL utilities, uses the environment variables
supported by libpq (see Section 34.15).

The environment variable PG_COLOR specifies whether to use color in diagnostic messages.
Possible values are always, auto and never.

NOTES
At the beginning of the backup, a checkpoint needs to be performed on the source server.
This can take some time (especially if the option --checkpoint=fast is not used), during
which pg_basebackup will 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,
except certain temporary files managed by PostgreSQL. But only regular files and
directories are copied, except that symbolic links used for tablespaces are preserved.
Symbolic links pointing to certain directories known to PostgreSQL are copied as empty
directories. Other symbolic links and special device files are skipped. See Section 53.4
for the precise details.

In plain format, tablespaces will be backed up to the same path they have on the source
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 is used, it is the user's responsibility to unpack each tar file before
starting a PostgreSQL server that uses the data. 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 (--format=tar) only works with server version 9.5 and later.

pg_basebackup will preserve group permissions for data files if group permissions are
enabled on the source cluster.

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 -X fetch | 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