25.1. Continuous Archiving and Point-in-Time Recovery (PITR) #

At all times, Postgres Pro 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.

Note

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, here we discuss some aspects of archiving WAL files. See pg_probackup for further reading.

25.1.1. Setting Up WAL Archiving #

In an abstract sense, a running Postgres Pro 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, Postgres Pro tries not to make any assumptions about how the archiving will be done. Instead, Postgres Pro lets the administrator specify a shell command or an archive library to be executed to copy a completed segment file to wherever it needs to go. This could be as simple as a shell command that uses cp, or it could invoke a complex C function — it's all up to you.

To enable WAL archiving, set the wal_level configuration parameter to replica or higher, archive_mode to on, specify the shell command to use in the archive_command configuration parameter or specify the library to use in the archive_library 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 Postgres Pro 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, Postgres Pro will assume that the file has been successfully archived, and will remove or recycle it. However, a nonzero status tells Postgres Pro that the file was not archived; it will try again periodically until it succeeds.

Another way to archive is to use a custom archive module as the archive_library. Since such modules are written in C, creating your own may require considerably more effort than writing a shell command. However, archive modules can be more performant than archiving via shell, and they will have access to many useful server resources. For more information about archive modules, see Chapter 52.

When the archive command is terminated by a signal (other than SIGTERM that is used as part of a server shutdown) or an error by the shell with an exit status greater than 125 (such as command not found), or if the archive function emits an ERROR or FATAL, the archiver process aborts and gets restarted by the postmaster. In such cases, the failure is not reported in pg_stat_archiver.

Archive commands and libraries 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 library to ensure that it does not overwrite an existing file.

In rare cases, Postgres Pro may attempt to re-archive a WAL file that was previously archived. For example, if the system crashes before the server makes a durable record of archival success, the server will attempt to archive the file again after restarting (provided archiving is still enabled). When an archive command or library encounters a pre-existing file, it should return a zero status or true, respectively, if the WAL file has identical contents to the pre-existing archive and the pre-existing archive is fully persisted to storage. If a pre-existing file contains different contents than the WAL file being archived, the archive command or library must return a nonzero status or false, respectively.

The example command above for Unix avoids overwriting a pre-existing archive 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 or library 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, Postgres Pro 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 archive command or library 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 or library, 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 Postgres Pro 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 or function 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 9.95.

When wal_level is minimal some SQL commands are optimized to avoid WAL logging. 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 and archive_library can be changed with a configuration file reload. If you are archiving via shell and 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.

25.1.2. Timelines #

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, Postgres Pro 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. For example, in the WAL file name 0000000100001234000055CD, the leading 00000001 is the timeline ID in hexadecimal. (Note that in other contexts, such as server log messages, timeline IDs are usually printed in decimal.)

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, Postgres Pro 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 to the latest timeline found in the archive. If you wish to recover to the timeline that was current when the base backup was taken or into a specific child timeline (that is, you want to return to some state that was itself generated after a recovery attempt), you need to specify current or the target timeline ID in recovery_target_timeline. You cannot recover into timelines that branched off earlier than the base backup.

25.1.3. Tips and Examples #

Some tips for configuring continuous archiving are given here.

25.1.3.1. Standalone Hot Backups #

It is possible to use Postgres Pro'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.)

25.1.3.2. Compressed Archive Logs #

If archive storage size is a concern, you can use gzip to compress the archive files:

archive_command = 'gzip < %p > /mnt/server/archivedir/%f.gz'

You will then need to use gunzip during recovery:

restore_command = 'gunzip < /mnt/server/archivedir/%f.gz > %p'

25.1.3.3. 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

Tip

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.

25.1.4. Caveats #

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 WAL is being replayed on a different machine. It can be dangerous even if the WAL 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 WAL files by turning off page snapshots using the full_page_writes parameter. (Read the notes and warnings in Chapter 28 before you do so.) Turning off page snapshots does not prevent use of the WAL 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.