At all times, PostgreSQL maintains a
write ahead log (WAL) in the pg_wal/
subdirectory of the cluster's data directory. The log records
every change made to the database's data files. This log exists
primarily for crash-safety purposes: if the system crashes, the
database can be restored to consistency by “replaying” the
log entries made since the last checkpoint. However, the existence
of the log makes it possible to use a third strategy for backing up
databases: we can combine a file-system-level backup with backup of
the WAL files. If recovery is needed, we restore the file system backup and
then replay from the backed-up WAL files to bring the system to a
current state. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:
We do not need a perfectly consistent file system backup as the starting point. Any internal inconsistency in the backup will be corrected by log replay (this is not significantly different from what happens during crash recovery). So we do not need a file system snapshot capability, just tar or a similar archiving tool.
Since we can combine an indefinitely long sequence of WAL files for replay, continuous backup can be achieved simply by continuing to archive the WAL files. This is particularly valuable for large databases, where it might not be convenient to take a full backup frequently.
It is not necessary to replay the WAL entries all the way to the end. We could stop the replay at any point and have a consistent snapshot of the database as it was at that time. Thus, this technique supports point-in-time recovery: it is possible to restore the database to its state at any time since your base backup was taken.
If we continuously feed the series of WAL files to another machine that has been loaded with the same base backup file, we have a warm standby system: at any point we can bring up the second machine and it will have a nearly-current copy of the database.
pg_dump and pg_dumpall do not produce file-system-level backups and cannot be used as part of a continuous-archiving solution. Such dumps are logical and do not contain enough information to be used by WAL replay.
As with the plain file-system-backup technique, this method can only support restoration of an entire database cluster, not a subset. Also, it requires a lot of archival storage: the base backup might be bulky, and a busy system will generate many megabytes of WAL traffic that have to be archived. Still, it is the preferred backup technique in many situations where high reliability is needed.
To recover successfully using continuous archiving (also called “online backup” by many database vendors), you need a continuous sequence of archived WAL files that extends back at least as far as the start time of your backup. So to get started, you should set up and test your procedure for archiving WAL files before you take your first base backup. Accordingly, we first discuss the mechanics of archiving WAL files.
In an abstract sense, a running PostgreSQL system produces an indefinitely long sequence of WAL records. The system physically divides this sequence into WAL segment files, which are normally 16MB apiece (although the segment size can be altered during initdb). The segment files are given numeric names that reflect their position in the abstract WAL sequence. When not using WAL archiving, the system normally creates just a few segment files and then “recycles” them by renaming no-longer-needed segment files to higher segment numbers. It's assumed that segment files whose contents precede the last checkpoint are no longer of interest and can be recycled.
When archiving WAL data, we need to capture the contents of each segment
file once it is filled, and save that data somewhere before the segment
file is recycled for reuse. Depending on the application and the
available hardware, there could be many different ways of “saving
the data somewhere”: we could copy the segment files to an NFS-mounted
directory on another machine, write them onto a tape drive (ensuring that
you have a way of identifying the original name of each file), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with flexibility,
PostgreSQL tries not to make any assumptions about how
the archiving will be done. Instead, PostgreSQL lets
the administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command could be
as simple as a cp, or it could invoke a complex shell
script — it's all up to you.
To enable WAL archiving, set the wal_level
configuration parameter to replica or higher,
archive_mode to on,
and specify the shell command to use in the archive_command configuration parameter. In practice
these settings will always be placed in the
postgresql.conf file.
In archive_command,
%p is replaced by the path name of the file to
archive, while %f is replaced by only the file name.
(The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Use %% if you need to embed an actual %
character in the command. The simplest useful command is something
like:
archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f' # Unix archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows
which will copy archivable WAL segments to the directory
/mnt/server/archivedir. (This is an example, not a
recommendation, and might not work on all platforms.) After the
%p and %f parameters have been replaced,
the actual command executed might look like this:
test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065
A similar command will be generated for each new file to be archived.
The archive command will be executed under the ownership of the same user that the PostgreSQL server is running as. Since the series of WAL files being archived contains effectively everything in your database, you will want to be sure that the archived data is protected from prying eyes; for example, archive into a directory that does not have group or world read access.
It is important that the archive command return zero exit status if and only if it succeeds. Upon getting a zero result, PostgreSQL will assume that the file has been successfully archived, and will remove or recycle it. However, a nonzero status tells PostgreSQL that the file was not archived; it will try again periodically until it succeeds.
The archive command should generally be designed to refuse to overwrite any pre-existing archive file. This is an important safety feature to preserve the integrity of your archive in case of administrator error (such as sending the output of two different servers to the same archive directory).
It is advisable to test your proposed archive command to ensure that it
indeed does not overwrite an existing file, and that it returns
nonzero status in this case.
The example command above for Unix ensures this by including a separate
test step. On some Unix platforms, cp has
switches such as -i that can be used to do the same thing
less verbosely, but you should not rely on these without verifying that
the right exit status is returned. (In particular, GNU cp
will return status zero when -i is used and the target file
already exists, which is not the desired behavior.)
While designing your archiving setup, consider what will happen if
the archive command fails repeatedly because some aspect requires
operator intervention or the archive runs out of space. For example, this
could occur if you write to tape without an autochanger; when the tape
fills, nothing further can be archived until the tape is swapped.
You should ensure that any error condition or request to a human operator
is reported appropriately so that the situation can be
resolved reasonably quickly. The pg_wal/ directory will
continue to fill with WAL segment files until the situation is resolved.
(If the file system containing pg_wal/ fills up,
PostgreSQL will do a PANIC shutdown. No committed
transactions will be lost, but the database will remain offline until
you free some space.)
The speed of the archiving command is unimportant as long as it can keep up
with the average rate at which your server generates WAL data. Normal
operation continues even if the archiving process falls a little behind.
If archiving falls significantly behind, this will increase the amount of
data that would be lost in the event of a disaster. It will also mean that
the pg_wal/ directory will contain large numbers of
not-yet-archived segment files, which could eventually exceed available
disk space. You are advised to monitor the archiving process to ensure that
it is working as you intend.
In writing your archive command, you should assume that the file names to
be archived can be up to 64 characters long and can contain any
combination of ASCII letters, digits, and dots. It is not necessary to
preserve the original relative path (%p) but it is necessary to
preserve the file name (%f).
Note that although WAL archiving will allow you to restore any
modifications made to the data in your PostgreSQL database,
it will not restore changes made to configuration files (that is,
postgresql.conf, pg_hba.conf and
pg_ident.conf), since those are edited manually rather
than through SQL operations.
You might wish to keep the configuration files in a location that will
be backed up by your regular file system backup procedures. See
Section 19.2 for how to relocate the
configuration files.
The archive command is only invoked on completed WAL segments. Hence,
if your server generates only little WAL traffic (or has slack periods
where it does so), there could be a long delay between the completion
of a transaction and its safe recording in archive storage. To put
a limit on how old unarchived data can be, you can set
archive_timeout to force the server to switch
to a new WAL segment file at least that often. Note that archived
files that are archived early due to a forced switch are still the same
length as completely full files. It is therefore unwise to set a very
short archive_timeout — it will bloat your archive
storage. archive_timeout settings of a minute or so are
usually reasonable.
Also, you can force a segment switch manually with
pg_switch_wal if you want to ensure that a
just-finished transaction is archived as soon as possible. Other utility
functions related to WAL management are listed in Table 9.79.
When wal_level is minimal some SQL commands
are optimized to avoid WAL logging, as described in Section 14.4.7. If archiving or streaming replication were
turned on during execution of one of these statements, WAL would not
contain enough information for archive recovery. (Crash recovery is
unaffected.) For this reason, wal_level can only be changed at
server start. However, archive_command can be changed with a
configuration file reload. If you wish to temporarily stop archiving,
one way to do it is to set archive_command to the empty
string ('').
This will cause WAL files to accumulate in pg_wal/ until a
working archive_command is re-established.
The easiest way to perform a base backup is to use the pg_basebackup tool. It can create a base backup either as regular files or as a tar archive. If more flexibility than pg_basebackup can provide is required, you can also make a base backup using the low level API (see Section 25.3.3).
It is not necessary to be concerned about the amount of time it takes
to make a base backup. However, if you normally run the
server with full_page_writes disabled, you might notice a drop
in performance while the backup runs since full_page_writes is
effectively forced on during backup mode.
To make use of the backup, you will need to keep all the WAL
segment files generated during and after the file system backup.
To aid you in doing this, the base backup process
creates a backup history file that is immediately
stored into the WAL archive area. This file is named after the first
WAL segment file that you need for the file system backup.
For example, if the starting WAL file is
0000000100001234000055CD the backup history file will be
named something like
0000000100001234000055CD.007C9330.backup. (The second
part of the file name stands for an exact position within the WAL
file, and can ordinarily be ignored.) Once you have safely archived
the file system backup and the WAL segment files used during the
backup (as specified in the backup history file), all archived WAL
segments with names numerically less are no longer needed to recover
the file system backup and can be deleted. However, you should
consider keeping several backup sets to be absolutely certain that
you can recover your data.
The backup history file is just a small text file. It contains the label string you gave to pg_basebackup, as well as the starting and ending times and WAL segments of the backup. If you used the label to identify the associated dump file, then the archived history file is enough to tell you which dump file to restore.
Since you have to keep around all the archived WAL files back to your last base backup, the interval between base backups should usually be chosen based on how much storage you want to expend on archived WAL files. You should also consider how long you are prepared to spend recovering, if recovery should be necessary — the system will have to replay all those WAL segments, and that could take awhile if it has been a long time since the last base backup.
The procedure for making a base backup using the low level APIs contains a few more steps than the pg_basebackup method, but is relatively simple. It is very important that these steps are executed in sequence, and that the success of a step is verified before proceeding to the next step.
Low level base backups can be made in a non-exclusive or an exclusive way. The non-exclusive method is recommended and the exclusive one is deprecated and will eventually be removed.
A non-exclusive low level backup is one that allows other concurrent backups to be running (both those started using the same backup API and those started using pg_basebackup).
Ensure that WAL archiving is enabled and working.
Connect to the server (it does not matter which database) as a user with rights to run pg_start_backup (superuser, or a user who has been granted EXECUTE on the function) and issue the command:
SELECT pg_start_backup('label', false, false);
where label is any string you want to use to uniquely
identify this backup operation. The connection
calling pg_start_backup must be maintained until the end of
the backup, or the backup will be automatically aborted.
By default, pg_start_backup can take a long time to finish.
This is because it performs a checkpoint, and the I/O
required for the checkpoint will be spread out over a significant
period of time, by default half your inter-checkpoint interval
(see the configuration parameter
checkpoint_completion_target). This is
usually what you want, because it minimizes the impact on query
processing. If you want to start the backup as soon as
possible, change the second parameter to true, which will
issue an immediate checkpoint using as much I/O as available.
The third parameter being false tells
pg_start_backup to initiate a non-exclusive base backup.
Perform the backup, using any convenient file-system-backup tool such as tar or cpio (not pg_dump or pg_dumpall). It is neither necessary nor desirable to stop normal operation of the database while you do this. See Section 25.3.3.3 for things to consider during this backup.
In the same connection as before, issue the command:
SELECT * FROM pg_stop_backup(false, true);
This terminates backup mode. On a primary, it also performs an automatic
switch to the next WAL segment. On a standby, it is not possible to
automatically switch WAL segments, so you may wish to run
pg_switch_wal on the primary to perform a manual
switch. The reason for the switch is to arrange for
the last WAL segment file written during the backup interval to be
ready to archive.
The pg_stop_backup will return one row with three
values. The second of these fields should be written to a file named
backup_label in the root directory of the backup. The
third field should be written to a file named
tablespace_map unless the field is empty. These files are
vital to the backup working and must be written byte for byte without
modification, which may require opening the file in binary mode.
Once the WAL segment files active during the backup are archived, you are
done. The file identified by pg_stop_backup's first return
value is the last segment that is required to form a complete set of
backup files. On a primary, if archive_mode is enabled and the
wait_for_archive parameter is true,
pg_stop_backup does not return until the last segment has
been archived.
On a standby, archive_mode must be always in order
for pg_stop_backup to wait.
Archiving of these files happens automatically since you have
already configured archive_command. In most cases this
happens quickly, but you are advised to monitor your archive
system to ensure there are no delays.
If the archive process has fallen behind
because of failures of the archive command, it will keep retrying
until the archive succeeds and the backup is complete.
If you wish to place a time limit on the execution of
pg_stop_backup, set an appropriate
statement_timeout value, but make note that if
pg_stop_backup terminates because of this your backup
may not be valid.
If the backup process monitors and ensures that all WAL segment files
required for the backup are successfully archived then the
wait_for_archive parameter (which defaults to true) can be set
to false to have
pg_stop_backup return as soon as the stop backup record is
written to the WAL. By default, pg_stop_backup will wait
until all WAL has been archived, which can take some time. This option
must be used with caution: if WAL archiving is not monitored correctly
then the backup might not include all of the WAL files and will
therefore be incomplete and not able to be restored.
The process for an exclusive backup is mostly the same as for a non-exclusive one, but it differs in a few key steps. This type of backup can only be taken on a primary and does not allow concurrent backups. Prior to PostgreSQL 9.6, this was the only low-level method available, but it is now recommended that all users upgrade their scripts to use non-exclusive backups if possible.
Ensure that WAL archiving is enabled and working.
Connect to the server (it does not matter which database) as a user with rights to run pg_start_backup (superuser, or a user who has been granted EXECUTE on the function) and issue the command:
SELECT pg_start_backup('label');
where label is any string you want to use to uniquely
identify this backup operation.
pg_start_backup creates a backup label file,
called backup_label, in the cluster directory with
information about your backup, including the start time and label string.
The function also creates a tablespace map file,
called tablespace_map, in the cluster directory with
information about tablespace symbolic links in pg_tblspc/ if
one or more such link is present. Both files are critical to the
integrity of the backup, should you need to restore from it.
By default, pg_start_backup can take a long time to finish.
This is because it performs a checkpoint, and the I/O
required for the checkpoint will be spread out over a significant
period of time, by default half your inter-checkpoint interval
(see the configuration parameter
checkpoint_completion_target). This is
usually what you want, because it minimizes the impact on query
processing. If you want to start the backup as soon as
possible, use:
SELECT pg_start_backup('label', true);
This forces the checkpoint to be done as quickly as possible.
Perform the backup, using any convenient file-system-backup tool such as tar or cpio (not pg_dump or pg_dumpall). It is neither necessary nor desirable to stop normal operation of the database while you do this. See Section 25.3.3.3 for things to consider during this backup.
Note that if the server crashes during the backup it may not be
possible to restart until the backup_label file has been
manually deleted from the PGDATA directory.
Again connect to the database as a user with rights to run pg_stop_backup (superuser, or a user who has been granted EXECUTE on the function), and issue the command:
SELECT pg_stop_backup();
This function terminates backup mode and performs an automatic switch to the next WAL segment. The reason for the switch is to arrange for the last WAL segment written during the backup interval to be ready to archive.
Once the WAL segment files active during the backup are archived, you are
done. The file identified by pg_stop_backup's result is
the last segment that is required to form a complete set of backup files.
If archive_mode is enabled,
pg_stop_backup does not return until the last segment has
been archived.
Archiving of these files happens automatically since you have
already configured archive_command. In most cases this
happens quickly, but you are advised to monitor your archive
system to ensure there are no delays.
If the archive process has fallen behind
because of failures of the archive command, it will keep retrying
until the archive succeeds and the backup is complete.
If you wish to place a time limit on the execution of
pg_stop_backup, set an appropriate
statement_timeout value, but make note that if
pg_stop_backup terminates because of this your backup
may not be valid.
Some file system backup tools emit warnings or errors
if the files they are trying to copy change while the copy proceeds.
When taking a base backup of an active database, this situation is normal
and not an error. However, you need to ensure that you can distinguish
complaints of this sort from real errors. For example, some versions
of rsync return a separate exit code for
“vanished source files”, and you can write a driver script to
accept this exit code as a non-error case. Also, some versions of
GNU tar return an error code indistinguishable from
a fatal error if a file was truncated while tar was
copying it. Fortunately, GNU tar versions 1.16 and
later exit with 1 if a file was changed during the backup,
and 2 for other errors. With GNU tar version 1.23 and
later, you can use the warning options --warning=no-file-changed
--warning=no-file-removed to hide the related warning messages.
Be certain that your backup includes all of the files under
the database cluster directory (e.g., /usr/local/pgsql/data).
If you are using tablespaces that do not reside underneath this directory,
be careful to include them as well (and be sure that your backup
archives symbolic links as links, otherwise the restore will corrupt
your tablespaces).
You should, however, omit from the backup the files within the
cluster's pg_wal/ subdirectory. This
slight adjustment is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
pg_wal/ is a symbolic link pointing to someplace outside
the cluster directory, which is a common setup anyway for performance
reasons. You might also want to exclude postmaster.pid
and postmaster.opts, which record information
about the running postmaster, not about the
postmaster which will eventually use this backup.
(These files can confuse pg_ctl.)
It is often a good idea to also omit from the backup the files
within the cluster's pg_replslot/ directory, so that
replication slots that exist on the master do not become part of the
backup. Otherwise, the subsequent use of the backup to create a standby
may result in indefinite retention of WAL files on the standby, and
possibly bloat on the master if hot standby feedback is enabled, because
the clients that are using those replication slots will still be connecting
to and updating the slots on the master, not the standby. Even if the
backup is only intended for use in creating a new master, copying the
replication slots isn't expected to be particularly useful, since the
contents of those slots will likely be badly out of date by the time
the new master comes on line.
The contents of the directories pg_dynshmem/,
pg_notify/, pg_serial/,
pg_snapshots/, pg_stat_tmp/,
and pg_subtrans/ (but not the directories themselves) can be
omitted from the backup as they will be initialized on postmaster startup.
If stats_temp_directory is set and is under the data
directory then the contents of that directory can also be omitted.
Any file or directory beginning with pgsql_tmp can be
omitted from the backup. These files are removed on postmaster start and
the directories will be recreated as needed.
pg_internal.init files can be omitted from the
backup whenever a file of that name is found. These files contain
relation cache data that is always rebuilt when recovering.
The backup label
file includes the label string you gave to pg_start_backup,
as well as the time at which pg_start_backup was run, and
the name of the starting WAL file. In case of confusion it is therefore
possible to look inside a backup file and determine exactly which
backup session the dump file came from. The tablespace map file includes
the symbolic link names as they exist in the directory
pg_tblspc/ and the full path of each symbolic link.
These files are not merely for your information; their presence and
contents are critical to the proper operation of the system's recovery
process.
It is also possible to make a backup while the server is
stopped. In this case, you obviously cannot use
pg_start_backup or pg_stop_backup, and
you will therefore be left to your own devices to keep track of which
backup is which and how far back the associated WAL files go.
It is generally better to follow the continuous archiving procedure above.
Okay, the worst has happened and you need to recover from your backup. Here is the procedure:
Stop the server, if it's running.
If you have the space to do so,
copy the whole cluster data directory and any tablespaces to a temporary
location in case you need them later. Note that this precaution will
require that you have enough free space on your system to hold two
copies of your existing database. If you do not have enough space,
you should at least save the contents of the cluster's pg_wal
subdirectory, as it might contain logs which
were not archived before the system went down.
Remove all existing files and subdirectories under the cluster data directory and under the root directories of any tablespaces you are using.
Restore the database files from your file system backup. Be sure that they
are restored with the right ownership (the database system user, not
root!) and with the right permissions. If you are using
tablespaces,
you should verify that the symbolic links in pg_tblspc/
were correctly restored.
Remove any files present in pg_wal/; these came from the
file system backup and are therefore probably obsolete rather than current.
If you didn't archive pg_wal/ at all, then recreate
it with proper permissions,
being careful to ensure that you re-establish it as a symbolic link
if you had it set up that way before.
If you have unarchived WAL segment files that you saved in step 2,
copy them into pg_wal/. (It is best to copy them,
not move them, so you still have the unmodified files if a
problem occurs and you have to start over.)
Create a recovery command file recovery.conf in the cluster
data directory (see Chapter 27). You might
also want to temporarily modify pg_hba.conf to prevent
ordinary users from connecting until you are sure the recovery was successful.
Start the server. The server will go into recovery mode and
proceed to read through the archived WAL files it needs. Should the
recovery be terminated because of an external error, the server can
simply be restarted and it will continue recovery. Upon completion
of the recovery process, the server will rename
recovery.conf to recovery.done (to prevent
accidentally re-entering recovery mode later) and then
commence normal database operations.
Inspect the contents of the database to ensure you have recovered to
the desired state. If not, return to step 1. If all is well,
allow your users to connect by restoring pg_hba.conf to normal.
The key part of all this is to set up a recovery configuration file that
describes how you want to recover and how far the recovery should
run. You can use recovery.conf.sample (normally
located in the installation's share/ directory) as a
prototype. The one thing that you absolutely must specify in
recovery.conf is the restore_command,
which tells PostgreSQL how to retrieve archived
WAL file segments. Like the archive_command, this is
a shell command string. It can contain %f, which is
replaced by the name of the desired log file, and %p,
which is replaced by the path name to copy the log file to.
(The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Write %% if you need to embed an actual %
character in the command. The simplest useful command is
something like:
restore_command = 'cp /mnt/server/archivedir/%f %p'
which will copy previously archived WAL segments from the directory
/mnt/server/archivedir. Of course, you can use something
much more complicated, perhaps even a shell script that requests the
operator to mount an appropriate tape.
It is important that the command return nonzero exit status on failure. The command will be called requesting files that are not present in the archive; it must return nonzero when so asked. This is not an error condition. An exception is that if the command was terminated by a signal (other than SIGTERM, which is used as part of a database server shutdown) or an error by the shell (such as command not found), then recovery will abort and the server will not start up.
Not all of the requested files will be WAL segment
files; you should also expect requests for files with a suffix of
.history. Also be aware that
the base name of the %p path will be different from
%f; do not expect them to be interchangeable.
WAL segments that cannot be found in the archive will be sought in
pg_wal/; this allows use of recent un-archived segments.
However, segments that are available from the archive will be used in
preference to files in pg_wal/.
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to the current point in time (or as
close as possible given the available WAL segments). Therefore, a normal
recovery will end with a “file not found” message, the exact text
of the error message depending upon your choice of
restore_command. You may also see an error message
at the start of recovery for a file named something like
00000001.history. This is also normal and does not
indicate a problem in simple recovery situations; see
Section 25.3.5 for discussion.
If you want to recover to some previous point in time (say, right before
the junior DBA dropped your main transaction table), just specify the
required stopping point in recovery.conf. You can specify
the stop point, known as the “recovery target”, either by
date/time, named restore point or by completion of a specific transaction
ID. As of this writing only the date/time and named restore point options
are very usable, since there are no tools to help you identify with any
accuracy which transaction ID to use.
The stop point must be after the ending time of the base backup, i.e.,
the end time of pg_stop_backup. You cannot use a base backup
to recover to a time when that backup was in progress. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
If recovery finds corrupted WAL data, recovery will
halt at that point and the server will not start. In such a case the
recovery process could be re-run from the beginning, specifying a
“recovery target” before the point of corruption so that recovery
can complete normally.
If recovery fails for an external reason, such as a system crash or
if the WAL archive has become inaccessible, then the recovery can simply
be restarted and it will restart almost from where it failed.
Recovery restart works much like checkpointing in normal operation:
the server periodically forces all its state to disk, and then updates
the pg_control file to indicate that the already-processed
WAL data need not be scanned again.
The ability to restore the database to a previous point in time creates some complexities that are akin to science-fiction stories about time travel and parallel universes. For example, in the original history of the database, suppose you dropped a critical table at 5:15PM on Tuesday evening, but didn't realize your mistake until Wednesday noon. Unfazed, you get out your backup, restore to the point-in-time 5:14PM Tuesday evening, and are up and running. In this history of the database universe, you never dropped the table. But suppose you later realize this wasn't such a great idea, and would like to return to sometime Wednesday morning in the original history. You won't be able to if, while your database was up-and-running, it overwrote some of the WAL segment files that led up to the time you now wish you could get back to. Thus, to avoid this, you need to distinguish the series of WAL records generated after you've done a point-in-time recovery from those that were generated in the original database history.
To deal with this problem, PostgreSQL has a notion of timelines. Whenever an archive recovery completes, a new timeline is created to identify the series of WAL records generated after that recovery. The timeline ID number is part of WAL segment file names so a new timeline does not overwrite the WAL data generated by previous timelines. It is in fact possible to archive many different timelines. While that might seem like a useless feature, it's often a lifesaver. Consider the situation where you aren't quite sure what point-in-time to recover to, and so have to do several point-in-time recoveries by trial and error until you find the best place to branch off from the old history. Without timelines this process would soon generate an unmanageable mess. With timelines, you can recover to any prior state, including states in timeline branches that you abandoned earlier.
Every time a new timeline is created, PostgreSQL creates a “timeline history” file that shows which timeline it branched off from and when. These history files are necessary to allow the system to pick the right WAL segment files when recovering from an archive that contains multiple timelines. Therefore, they are archived into the WAL archive area just like WAL segment files. The history files are just small text files, so it's cheap and appropriate to keep them around indefinitely (unlike the segment files which are large). You can, if you like, add comments to a history file to record your own notes about how and why this particular timeline was created. Such comments will be especially valuable when you have a thicket of different timelines as a result of experimentation.
The default behavior of recovery is to recover along the same timeline
that was current when the base backup was taken. If you wish to recover
into some child timeline (that is, you want to return to some state that
was itself generated after a recovery attempt), you need to specify the
target timeline ID in recovery.conf. You cannot recover into
timelines that branched off earlier than the base backup.
Some tips for configuring continuous archiving are given here.
It is possible to use PostgreSQL's backup facilities to produce standalone hot backups. These are backups that cannot be used for point-in-time recovery, yet are typically much faster to backup and restore than pg_dump dumps. (They are also much larger than pg_dump dumps, so in some cases the speed advantage might be negated.)
As with base backups, the easiest way to produce a standalone
hot backup is to use the pg_basebackup
tool. If you include the -X parameter when calling
it, all the write-ahead log required to use the backup will be
included in the backup automatically, and no special action is
required to restore the backup.
If more flexibility in copying the backup files is needed, a lower
level process can be used for standalone hot backups as well.
To prepare for low level standalone hot backups, make sure
wal_level is set to
replica or higher, archive_mode to
on, and set up an archive_command that performs
archiving only when a switch file exists. For example:
archive_command = 'test ! -f /var/lib/pgsql/backup_in_progress || (test ! -f /var/lib/pgsql/archive/%f && cp %p /var/lib/pgsql/archive/%f)'
This command will perform archiving when
/var/lib/pgsql/backup_in_progress exists, and otherwise
silently return zero exit status (allowing PostgreSQL
to recycle the unwanted WAL file).
With this preparation, a backup can be taken using a script like the following:
touch /var/lib/pgsql/backup_in_progress
psql -c "select pg_start_backup('hot_backup');"
tar -cf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
psql -c "select pg_stop_backup();"
rm /var/lib/pgsql/backup_in_progress
tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
The switch file /var/lib/pgsql/backup_in_progress is
created first, enabling archiving of completed WAL files to occur.
After the backup the switch file is removed. Archived WAL files are
then added to the backup so that both base backup and all required
WAL files are part of the same tar file.
Please remember to add error handling to your backup scripts.
If archive storage size is a concern, you can use gzip to compress the archive files:
archive_command = 'gzip < %p > /var/lib/pgsql/archive/%f'
You will then need to use gunzip during recovery:
restore_command = 'gunzip < /mnt/server/archivedir/%f > %p'
archive_command Scripts
Many people choose to use scripts to define their
archive_command, so that their
postgresql.conf entry looks very simple:
archive_command = 'local_backup_script.sh "%p" "%f"'
Using a separate script file is advisable any time you want to use more than a single command in the archiving process. This allows all complexity to be managed within the script, which can be written in a popular scripting language such as bash or perl.
Examples of requirements that might be solved within a script include:
Copying data to secure off-site data storage
Batching WAL files so that they are transferred every three hours, rather than one at a time
Interfacing with other backup and recovery software
Interfacing with monitoring software to report errors
When using an archive_command script, it's desirable
to enable logging_collector.
Any messages written to stderr from the script will then
appear in the database server log, allowing complex configurations to
be diagnosed easily if they fail.
At this writing, there are several limitations of the continuous archiving technique. These will probably be fixed in future releases:
If a CREATE DATABASE
command is executed while a base backup is being taken, and then
the template database that the CREATE DATABASE copied
is modified while the base backup is still in progress, it is
possible that recovery will cause those modifications to be
propagated into the created database as well. This is of course
undesirable. To avoid this risk, it is best not to modify any
template databases while taking a base backup.
CREATE TABLESPACE commands are WAL-logged with the literal absolute path, and will therefore be replayed as tablespace creations with the same absolute path. This might be undesirable if the log is being replayed on a different machine. It can be dangerous even if the log is being replayed on the same machine, but into a new data directory: the replay will still overwrite the contents of the original tablespace. To avoid potential gotchas of this sort, the best practice is to take a new base backup after creating or dropping tablespaces.
It should also be noted that the default WAL
format is fairly bulky since it includes many disk page snapshots.
These page snapshots are designed to support crash recovery, since
we might need to fix partially-written disk pages. Depending on
your system hardware and software, the risk of partial writes might
be small enough to ignore, in which case you can significantly
reduce the total volume of archived logs by turning off page
snapshots using the full_page_writes
parameter. (Read the notes and warnings in Chapter 30
before you do so.) Turning off page snapshots does not prevent
use of the logs for PITR operations. An area for future
development is to compress archived WAL data by removing
unnecessary page copies even when full_page_writes is
on. In the meantime, administrators might wish to reduce the number
of page snapshots included in WAL by increasing the checkpoint
interval parameters as much as feasible.