lt_basebackup — take a base backup of a LightDB cluster
lt_basebackup [option...]
lt_basebackup is used to take a base backup of a running LightDB 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 23.3) and as the starting point for a log-shipping or streaming-replication standby server (see Section 24.2).
lt_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 lt_dump must be used.
The backup is made over a regular LightDB
connection that uses the replication protocol. The connection must be made
with a user ID that has REPLICATION permissions
(see Section 19.2) or is 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 provide at
least one walsender for the backup plus one for WAL streaming (if used).
There can be multiple lt_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.
lt_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 lt_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:
The backup history file is not created in the database cluster backed up.
lt_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, lt_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.
If the standby is promoted to be primary during 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 primary and
not to use a tool like pg_compresslog as
archive_command to remove full-page writes from WAL files.
Whenever lt_basebackup is taking a base
backup, the server's pg_stat_progress_basebackup
view will report the progress of the backup.
See Section 25.4.5 for details.
The following command-line options control the location and format of the output:
-D directory--pgdata=directorySets the target directory to write the output to. lt_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:
pplain
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.
ttar
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 lightdb.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 lightdb.auto.conf file will record the connection
settings and, if specified, the replication slot
that lt_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 lt_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:
nnoneDon't include write-ahead logs in the backup.
ffetchThe 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.
sstreamStream 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 lt_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|spreadSets checkpoint mode to fast (immediate) or spread (the default) (see Section 23.3.3).
-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
“lt_basebackup base backup” will be used.
-n--no-clean
By default, when lt_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, lt_basebackup will wait for all files
to be written safely to disk. This option causes
lt_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 lt_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--verboseEnables 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.
lt_verifybackup can be used to check the integrity of a backup against the backup manifest.
--manifest-force-encodeForces 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-manifestDisables 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 lt_verifybackup. 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-slotPrevents 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-checksumsDisables 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=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_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 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.
-s interval--status-interval=intervalSpecifies 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=usernameSpecifies 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--passwordForces lt_basebackup to prompt for a password before connecting to the source server.
This option is never essential, since
lt_basebackup will automatically prompt
for a password if the server demands password authentication.
However, lt_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--versionPrints the lt_basebackup version and exits.
-?--helpShows help about lt_basebackup command line arguments, and exits.
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.
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 lt_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 LightDB. 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 LightDB are copied as empty directories. Other symbolic links and special device files are skipped. See Section 52.4 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 LightDB 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.
lt_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.
lt_basebackup will preserve group permissions for data files if group permissions are enabled on the source cluster.
To create a base backup of the server at mydbserver
and store it in the local directory
/usr/local/pgsql/data:
$lt_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:
$lt_basebackup -D backup -Ft -z -P
To create a backup of a single-tablespace local database and compress this with bzip2:
$lt_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:
$lt_basebackup -D backup/data -T /opt/ts=$(pwd)/backup/ts