backup As with everything that contains valuable data, PostgreSQL databases should be backed up regularly. While the procedure is essentially simple, it is important to have a basic understanding of the underlying techniques and assumptions. There are three fundamentally different approaches to backing up PostgreSQL data: SQL dump File system level backup Continuous Archiving Each has its own strengths and weaknesses. The idea behind the SQL-dump method is to generate a text file with SQL commands that, when fed back to the server, will recreate the database in the same state as it was at the time of the dump. PostgreSQL provides the utility program for this purpose. The basic usage of this command is: pg_dump dbname > outfile As you see, pg_dump writes its results to the standard output. We will see below how this can be useful. pg_dump is a regular PostgreSQL client application (albeit a particularly clever one). This means that you can do this backup procedure from any remote host that has access to the database. But remember that pg_dump does not operate with special permissions. In particular, it must have read access to all tables that you want to back up, so in practice you almost always have to run it as a database superuser. To specify which database server pg_dump should contact, use the command line options -h host and -p port. The default host is the local host or whatever your PGHOST environment variable specifies. Similarly, the default port is indicated by the PGPORT environment variable or, failing that, by the compiled-in default. (Conveniently, the server will normally have the same compiled-in default.) As any other PostgreSQL client application, pg_dump will by default connect with the database user name that is equal to the current operating system user name. To override this, either specify the -U option or set the environment variable PGUSER. Remember that pg_dump connections are subject to the normal client authentication mechanisms (which are described in ). Dumps created by pg_dump are internally consistent, that is, updates to the database while pg_dump is running will not be in the dump. pg_dump does not block other operations on the database while it is working. (Exceptions are those operations that need to operate with an exclusive lock, such as VACUUM FULL.) When your database schema relies on OIDs (for instance as foreign keys) you must instruct pg_dump to dump the OIDs as well. To do this, use the -o command line option. The text files created by pg_dump are intended to be read in by the psql program. The general command form to restore a dump is psql dbname < infile where infile is what you used as outfile for the pg_dump command. The database dbname will not be created by this command, you must create it yourself from template0 before executing psql (e.g., with createdb -T template0 dbname). psql supports options similar to pg_dump for controlling the database server location and the user name. See 's reference page for more information. Not only must the target database already exist before starting to run the restore, but so must all the users who own objects in the dumped database or were granted permissions on the objects. If they do not, then the restore will fail to recreate the objects with the original ownership and/or permissions. (Sometimes this is what you want, but usually it is not.) Once restored, it is wise to run on each database so the optimizer has useful statistics. An easy way to do this is to run vacuumdb -a -z to VACUUM ANALYZE all databases; this is equivalent to running VACUUM ANALYZE manually. The ability of pg_dump and psql to write to or read from pipes makes it possible to dump a database directly from one server to another; for example: pg_dump -h host1 dbname | psql -h host2 dbname The dumps produced by pg_dump are relative to template0. This means that any languages, procedures, etc. added to template1 will also be dumped by pg_dump. As a result, when restoring, if you are using a customized template1, you must create the empty database from template0, as in the example above. For advice on how to load large amounts of data into PostgreSQL efficiently, refer to . The above mechanism is cumbersome and inappropriate when backing up an entire database cluster. For this reason the program is provided. pg_dumpall backs up each database in a given cluster, and also preserves cluster-wide data such as users and groups. The basic usage of this command is: pg_dumpall > outfile The resulting dump can be restored with psql: psql -f infile postgres (Actually, you can specify any existing database name to start from, but if you are reloading in an empty cluster then postgres should generally be used.) It is always necessary to have database superuser access when restoring a pg_dumpall dump, as that is required to restore the user and group information. Since PostgreSQL allows tables larger than the maximum file size on your system, it can be problematic to dump such a table to a file, since the resulting file will likely be larger than the maximum size allowed by your system. Since pg_dump can write to the standard output, you can just use standard Unix tools to work around this possible problem. You can use your favorite compression program, for example gzip. pg_dump dbname | gzip > filename.gz Reload with createdb dbname gunzip -c filename.gz | psql dbname or cat filename.gz | gunzip | psql dbname The split command allows you to split the output into pieces that are acceptable in size to the underlying file system. For example, to make chunks of 1 megabyte: pg_dump dbname | split -b 1m - filename Reload with createdb dbname cat filename* | psql dbname If PostgreSQL was built on a system with the zlib compression library installed, the custom dump format will compress data as it writes it to the output file. This will produce dump file sizes similar to using gzip, but it has the added advantage that tables can be restored selectively. The following command dumps a database using the custom dump format: pg_dump -Fc dbname > filename A custom-format dump is not a script for psql, but instead must be restored with pg_restore. See the and reference pages for details. An alternative backup strategy is to directly copy the files that PostgreSQL uses to store the data in the database. In it is explained where these files are located, but you have probably found them already if you are interested in this method. You can use whatever method you prefer for doing usual file system backups, for example tar -cf backup.tar /usr/local/pgsql/data There are two restrictions, however, which make this method impractical, or at least inferior to the pg_dump method: The database server must be shut down in order to get a usable backup. Half-way measures such as disallowing all connections will not work (mainly because tar and similar tools do not take an atomic snapshot of the state of the file system at a point in time). Information about stopping the server can be found in . Needless to say that you also need to shut down the server before restoring the data. If you have dug into the details of the file system layout of the database, you may be tempted to try to back up or restore only certain individual tables or databases from their respective files or directories. This will not work because the information contained in these files contains only half the truth. The other half is in the commit log files pg_clog/*, which contain the commit status of all transactions. A table file is only usable with this information. Of course it is also impossible to restore only a table and the associated pg_clog data because that would render all other tables in the database cluster useless. So file system backups only work for complete restoration of an entire database cluster. An alternative file-system backup approach is to make a consistent snapshot of the data directory, if the file system supports that functionality (and you are willing to trust that it is implemented correctly). The typical procedure is to make a frozen snapshot of the volume containing the database, then copy the whole data directory (not just parts, see above) from the snapshot to a backup device, then release the frozen snapshot. This will work even while the database server is running. However, a backup created in this way saves the database files in a state where the database server was not properly shut down; therefore, when you start the database server on the backed-up data, it will think the server had crashed and replay the WAL log. This is not a problem, just be aware of it (and be sure to include the WAL files in your backup). If your database is spread across multiple file systems, there may not be any way to obtain exactly-simultaneous frozen snapshots of all the volumes. For example, if your data files and WAL log are on different disks, or if tablespaces are on different file systems, it might not be possible to use snapshot backup because the snapshots must be simultaneous. Read your file system documentation very carefully before trusting to the consistent-snapshot technique in such situations. The safest approach is to shut down the database server for long enough to establish all the frozen snapshots. Another option is to use rsync to perform a file system backup. This is done by first running rsync while the database server is running, then shutting down the database server just long enough to do a second rsync. The second rsync will be much quicker than the first, because it has relatively little data to transfer, and the end result will be consistent because the server was down. This method allows a file system backup to be performed with minimal downtime. Note that a file system backup will not necessarily be smaller than an SQL dump. On the contrary, it will most likely be larger. (pg_dump does not need to dump the contents of indexes for example, just the commands to recreate them.) continuous archiving point-in-time recovery PITR At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log describes 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 backup and then replay from the backed-up WAL files to bring the backup up to current time. 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 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 don't need file system snapshot capability, just tar or a similar archiving tool. Since we can string together 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 may not be convenient to take a full backup frequently. There is nothing that says we have 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 hot standby system: at any point we can bring up the second machine and it will have a nearly-current copy of the database. 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 may 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 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 restoring the file with its original file name), 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 configuration parameter, which in practice will always be placed in the postgresql.conf file. In this string, any %p is replaced by the absolute path of the file to archive, while any %f is replaced by the file name only. 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. If you are concerned about being able to recover right up to the current instant, you may want to take additional steps to ensure that the current, partially-filled WAL segment is also copied someplace. This is particularly important if your server generates only little WAL traffic (or has slack periods where it does so), since it could take a long time before a WAL segment file is completely filled and ready to archive. One possible way to handle this is to set up a cron job that periodically (once a minute, perhaps) identifies the current WAL segment file and saves it someplace safe. Then the combination of the archived WAL segments and the saved current segment will be enough to ensure you can always restore to within a minute of current time. This behavior is not presently built into PostgreSQL because we did not want to complicate the definition of the by requiring it to keep track of successively archived, but different, copies of the same WAL file. The is only invoked on completed WAL segments. Except in the case of retrying a failure, it will be called only once for any given file name. Another way to limit your exposure to data loss is to call pg_switch_xlog() periodically, such as once a minute. This function forces the current WAL segment file to be completed and made available to the archiving command. This approach does not work well for extremely short update intervals, however, since copying a new 16MB segment file every few seconds is expensive. 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 full 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 for how to relocate the configuration files. The procedure for making a base backup is relatively simple: Ensure that WAL archiving is enabled and working. 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. 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. Again connect to the database as a superuser, and issue the command SELECT pg_stop_backup(); This should return successfully. Once the WAL segment files used during the backup are archived as part of normal database activity, 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. 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 consider it an error if a file is changed while tar is copying it. There does not seem to be any very convenient way to distinguish this error from other types of errors, other than manual inspection of tar's messages. GNU tar is therefore not the best tool for making base backups. 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. You must however be quite sure that these operations are carried out in sequence and do not overlap. 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 this 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. 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 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. Clean out 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 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 may want to verify that the symbolic links in pg_tblspc/ were correctly restored. 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 re-create it, and be sure to re-create the subdirectory pg_xlog/archive_status/ as well. 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.) Create a recovery command file recovery.conf in the cluster data directory (see ). You may also want to temporarily modify pg_hba.conf to prevent ordinary users from connecting until you are sure the recovery has worked. Start the server. The server will go into recovery mode and proceed to read through the archived WAL files it needs. 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. 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 set up 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 absolute path to copy the log file to. 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. The stop point must be after the ending time of the base backup (the time of pg_stop_backup). You cannot use a base backup to recover to a time when that backup was still going on. (To recover to such a time, you must go back to your previous base backup and roll forward from there.) These settings can only be made in the recovery.conf file, and apply only for the duration of the recovery. They must be reset for any subsequent recovery you wish to perform. They cannot be changed once recovery has begun. restore_command (string) The shell command to execute to retrieve an archived segment of the WAL file series. This parameter is required. Any %f in the string is replaced by the name of the file to retrieve from the archive, and any %p is replaced by the absolute path to copy it to on the server. Write %% to embed an actual % character in the command. It is important for the command to return a zero exit status if and only if it succeeds. The command will be asked for file names that are not present in the archive; it must return nonzero when so asked. Examples: restore_command = 'cp /mnt/server/archivedir/%f "%p"' restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows recovery_target_time (timestamp) This parameter specifies the time stamp up to which recovery will proceed. At most one of recovery_target_time and can be specified. The default is to recover to the end of the WAL log. The precise stopping point is also influenced by . recovery_target_xid (string) This parameter specifies the transaction ID up to which recovery will proceed. Keep in mind that while transaction IDs are assigned sequentially at transaction start, transactions can complete in a different numeric order. The transactions that will be recovered are those that committed before (and optionally including) the specified one. At most one of recovery_target_xid and can be specified. The default is to recover to the end of the WAL log. The precise stopping point is also influenced by . recovery_target_inclusive (boolean) Specifies whether we stop just after the specified recovery target (true), or just before the recovery target (false). Applies to both and , whichever one is specified for this recovery. This indicates whether transactions having exactly the target commit time or ID, respectively, will be included in the recovery. Default is true. recovery_target_timeline (string) Specifies recovering into a particular timeline. The default is to recover along the same timeline that was current when the base backup was taken. You would only need to set this parameter in complex re-recovery situations, where you need to return to a state that itself was reached after a point-in-time recovery. See for discussion. 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. 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 parameter. (Read the notes and warnings in 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. upgrading version compatibility This section discusses how to migrate your database data from one PostgreSQL release to a newer one. The software installation procedure per se is not the subject of this section; those details are in . As a general rule, the internal data storage format is subject to change between major releases of PostgreSQL (where the number after the first dot changes). This does not apply to different minor releases under the same major release (where the number after the second dot changes); these always have compatible storage formats. For example, releases 7.2.1, 7.3.2, and 7.4 are not compatible, whereas 7.2.1 and 7.2.2 are. When you update between compatible versions, you can simply replace the executables and reuse the data directory on disk. Otherwise you need to back up your data and restore it on the new server. This has to be done using pg_dump; file system level backup methods obviously won't work. There are checks in place that prevent you from using a data directory with an incompatible version of PostgreSQL, so no great harm can be done by trying to start the wrong server version on a data directory. It is recommended that you use the pg_dump and pg_dumpall programs from the newer version of PostgreSQL, to take advantage of any enhancements that may have been made in these programs. Current releases of the dump programs can read data from any server version back to 7.0. The least downtime can be achieved by installing the new server in a different directory and running both the old and the new servers in parallel, on different ports. Then you can use something like pg_dumpall -p 5432 | psql -d postgres -p 6543 to transfer your data. Or use an intermediate file if you want. Then you can shut down the old server and start the new server at the port the old one was running at. You should make sure that the old database is not updated after you run pg_dumpall, otherwise you will obviously lose that data. See for information on how to prohibit access. In practice you probably want to test your client applications on the new setup before switching over completely. This is another reason for setting up concurrent installations of old and new versions. If you cannot or do not want to run two servers in parallel you can do the backup step before installing the new version, bring down the server, move the old version out of the way, install the new version, start the new server, restore the data. For example: pg_dumpall > backup pg_ctl stop mv /usr/local/pgsql /usr/local/pgsql.old cd ~/postgresql-&version; gmake install initdb -D /usr/local/pgsql/data postgres -D /usr/local/pgsql/data psql -f backup postgres See about ways to start and stop the server and other details. The installation instructions will advise you of strategic places to perform these steps. When you move the old installation out of the way it may no longer be perfectly usable. Some of the executable programs contain absolute paths to various installed programs and data files. This is usually not a big problem but if you plan on using two installations in parallel for a while you should assign them different installation directories at build time. (This problem is rectified in PostgreSQL 8.0 and later, but you need to be wary of moving older installations.)