23.3.1. Setting up WAL archiving

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 size can be altered when building PostgreSQL). 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 a segment file whose contents precede the checkpoint-before-last is no longer of interest and can be recycled.

When archiving WAL data, we want 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 as much flexibility as possible, 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.

The shell command to use is specified by the archive_command configuration parameter, which in practice will always be placed in the postgresql.conf file. In this string, any %p is replaced by the path name of the file to archive, while any %f is replaced by the file name only. (The path name is relative to the working directory of the server, 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

archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'

which will copy archivable WAL segments to the directory /mnt/server/archivedir. (This is an example, not a recommendation, and may not work on all platforms.)

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 succeeded. Upon getting a zero result, PostgreSQL will assume that the WAL segment 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. We have found that cp -i does this correctly on some platforms but not others. If the chosen command does not itself handle this case correctly, you should add a command to test for pre-existence of the archive file. For example, something like

archive_command = 'test ! -f .../%f && cp %p .../%f'

works correctly on most Unix variants.

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 relatively quickly. The pg_xlog/ directory will continue to fill with WAL segment files until the situation is resolved.

The speed of the archiving command is not important, so 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_xlog/ 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 may be up to 64 characters long and may contain any combination of ASCII letters, digits, and dots. It is not necessary to remember the original relative path (%p) but it is necessary to remember 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 may wish to keep the configuration files in a location that will be backed up by your regular file system backup procedures. See Section 17.2 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 ended 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_xlog, if you want to ensure that a just-finished transaction is archived immediately. Other utility functions related to WAL management are listed in Table 9-47.

23.3.2. Making a Base Backup

The procedure for making a base backup is relatively simple:

1. Ensure that WAL archiving is enabled and working.

2. Connect to the database as a superuser, and issue the command

   SELECT pg_start_backup('label');

   where label is any string you want to use to uniquely identify this backup operation. (One good practice is to use the full path where you intend to put the backup dump file.) pg_start_backup creates a backup label file, called backup_label, in the cluster directory with information about your backup.

   It does not matter which database within the cluster you connect to to issue this command. You can ignore the result returned by the function; but if it reports an error, deal with that before proceeding.

3. Perform the backup, using any convenient file-system-backup tool such as tar or cpio. It is neither necessary nor desirable to stop normal operation of the database while you do this.

4. Again connect to the database as a superuser, and issue the command

   SELECT pg_stop_backup();

   This terminates the backup mode and performs an automatic switch to the next WAL segment. The reason for the switch is to arrange that the last WAL segment file written during the backup interval is immediately ready to archive.

5. Once the WAL segment files used during the backup are archived, you are done. The file identified by pg_stop_backup's result is the last segment that needs to be archived to complete the backup. Archival of these files will happen automatically, since you have already configured archive_command. In many cases, this happens fairly quickly, but you are advised to monitor your archival system to ensure this has taken place so that you can be certain you have a complete backup.

Some backup tools that you might wish to use emit warnings or errors if the files they are trying to copy change while the copy proceeds. This situation is normal, and not an error, when taking a base backup of an active database; so 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 exits with 1 if a file was changed during the backup, and 2 for other errors.

It is not necessary to be very concerned about the amount of time elapsed between pg_start_backup and the start of the actual backup, nor between the end of the backup and pg_stop_backup; a few minutes' delay won't hurt anything. (However, if you normally run the server with full_page_writes disabled, you may notice a drop in performance between pg_start_backup and pg_stop_backup, since full_page_writes is effectively forced on during backup mode.) You must ensure that these steps are carried out in sequence without any possible overlap, or you will invalidate the backup.

Be certain that your backup dump includes all of the files underneath 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 dump archives symbolic links as links, otherwise the restore will mess up your tablespaces).

You may, however, omit from the backup dump the files within the pg_xlog/ subdirectory of the cluster directory. This slight complication is worthwhile because it reduces the risk of mistakes when restoring. This is easy to arrange if pg_xlog/ is a symbolic link pointing to someplace outside the cluster directory, which is a common setup anyway for performance reasons.

To make use of the backup, you will need to keep around all the WAL segment files generated during and after the file system backup. To aid you in doing this, the pg_stop_backup function 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 to have to make use of the backup. For example, if the starting WAL file is 0000000100001234000055CD the backup history file will be named something like 0000000100001234000055CD.007C9330.backup. (The second number in 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 may 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_start_backup, as well as the starting and ending times and WAL segments of the backup. If you used the label to identify where the associated dump file is kept, then the archived history file is enough to tell you which dump file to restore, should you need to do so.

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.

It's also worth noting that the pg_start_backup function makes a file named backup_label in the database cluster directory, which is then removed again by pg_stop_backup. This file will of course be archived as a part of your backup dump file. 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 will therefore be possible to look inside a backup dump file and determine exactly which backup session the dump file came from.

It is also possible to make a backup dump 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 dump is which and how far back the associated WAL files go. It is generally better to follow the continuous archiving procedure above.

23.3.3. Recovering using a Continuous Archive Backup

Okay, the worst has happened and you need to recover from your backup. Here is the procedure:

1. Stop the server, if it's running.

2. 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 need at the least to copy the contents of the pg_xlog subdirectory of the cluster data directory, as it may contain logs which were not archived before the system went down.

3. Clean out all existing files and subdirectories under the cluster data directory and under the root directories of any tablespaces you are using.

4. Restore the database files from your backup dump. Be careful 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.

5. Remove any files present in pg_xlog/; these came from the backup dump and are therefore probably obsolete rather than current. If you didn't archive pg_xlog/ at all, then recreate it, and be sure to recreate the subdirectory pg_xlog/archive_status/ as well.

6. If you had unarchived WAL segment files that you saved in step 2, copy them into pg_xlog/. (It is best to copy them, not move them, so that you still have the unmodified files if a problem occurs and you have to start over.)

7. Create a recovery command file recovery.conf in the cluster data directory (see Recovery Settings). You may also want to temporarily modify pg_hba.conf to prevent ordinary users from connecting until you are sure the recovery has worked.

8. 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 in case of a crash later) and then commence normal database operations.

9. Inspect the contents of the database to ensure you have recovered to where you want to be. If not, return to step 1. If all is well, let in your users by restoring pg_hba.conf to normal.

The key part of all this is to setup a recovery command file that describes how you want to recover and how far the recovery should run. You can use recovery.conf.sample (normally installed in the installation 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 get back archived WAL file segments. Like the archive_command, this is a shell command string. It may 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 working directory of the server, 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. You could of course 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 asked for log files that are not present in the archive; it must return nonzero when so asked. This is not an error condition. Be aware also 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_xlog/; 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_xlog/. The system will not overwrite the existing contents of pg_xlog/ when retrieving archived files.

Normally, recovery will proceed through all available WAL segments, thereby restoring the database to the current point in time (or as close as we can get given the available WAL segments). But 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 or by completion of a specific transaction ID. As of this writing only the date/time option is very usable, since there are no tools to help you identify with any accuracy which transaction ID to use.

If recovery finds a corruption in the WAL data then recovery will complete 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.

23.3.4. 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. In the original history of the database, perhaps you dropped a critical table at 5:15PM on Tuesday evening. 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 at all. But suppose you later realize this wasn't such a great idea after all, and would like to return to some later point in the original history. You won't be able to if, while your database was up-and-running, it overwrote some of the sequence of WAL segment files that led up to the time you now wish you could get back to. So you really want 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 these problems, PostgreSQL has a notion of timelines. Each time you recover to a point-in-time earlier than the end of the WAL sequence, a new timeline is created to identify the series of WAL records generated after that recovery. (If recovery proceeds all the way to the end of WAL, however, we do not start a new timeline: we just extend the existing one.) The timeline ID number is part of WAL segment file names, and 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 later abandoned.

Each 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 make your own notes about how and why this particular timeline came to be. 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 want 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.

23.3.5. Caveats

At this writing, there are several limitations of the continuous archiving technique. These will probably be fixed in future releases:

• Operations on hash indexes are not presently WAL-logged, so replay will not update these indexes. The recommended workaround is to manually REINDEX each such index after completing a recovery operation.

• 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 may need to fix partially-written disk pages. Depending on your system hardware and software, the risk of partial writes may 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 27 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 may wish to reduce the number of page snapshots included in WAL by increasing the checkpoint interval parameters as much as feasible.