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

23.3.1. Setting Up WAL Archiving

This section describes PITR for PostgreSQL. Because Coordinator and Datanode of Postgres-XC are essentially PostgreSQLserver, you can set up WAL archiving for each Coordinator and Datanode manually.

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 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 segment files whose contents precede the checkpoint-before-last 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 archive (or hot_standby), 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 % characterin the command. The simplest useful command is something like:

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

cp -i pg_xlog/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065 </dev/null

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. On many Unix platforms, cp -i causes copy to prompt before overwriting a file, and < /dev/null causes the prompt (and overwriting) to fail. If your platform does not support this behavior, you should add a command to test for the existence of the archive file. For example, something like:

archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%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 reasonably quickly. The pg_xlog/ directory will continue to fill with WAL segment files until the situation is resolved. (If the file system containing pg_xlog/ 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_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 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 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 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_xlog 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 9-56.

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_xlog/ until a working archive_command is re-established.

23.3.2. Making a Base Backup

This section describes how to make a base backup of single Datanode or Coordinator. Please note that you should take base backup of all the Datanodes and Coordinators. Also please note that you don't have to take base backups at exactly the same time. You may take base backup of each Datanode or Coordinator one after another, or you may take some of them at the same time. You don't have to do it at exactly the same time.

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, including the start time and label string.

   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.

   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.

3. 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.

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 for the last WAL segment file written during the backup interval to be ready to archive.

5. 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.

You can also use the pg_basebackup tool to take the backup, instead of manually copying the files. This tool will do the equivalent of pg_start_backup(), copy and pg_stop_backup() steps automatically, and transfers the backup over a regular PostgreSQL connection using the replication protocol, instead of requiring file system level access. pg_basebackup does not interfere with file system level backups taken using pg_start_backup()/pg_stop_backup().

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.

It is not necessary to be 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 might 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 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 dump archives symbolic links as links, otherwise the restore will corrupt your tablespaces).

You can, however, omit from the backup dump the files within the cluster's pg_xlog/ subdirectory. This slight adjustment 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 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 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_start_backup, 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.

It's also worth noting that the pg_start_backup function makes a file named backup_label in the database cluster directory, which is removed 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 is therefore 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

This section describes recovering with continuous archive backup for PostgreSQL. Because Coordinator and Datanode of Postgres-XC are essentially PostgreSQLserver, you can do this for each Coordinator and Datanode manually.

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 should at least save the contents of the cluster's pg_xlog subdirectory, as it might contain logs which were not archived before the system went down.

3. Remove 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 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.

5. Remove any files present in pg_xlog/; these came from the file system backup and are therefore probably obsolete rather than current. If you didn't archive pg_xlog/ 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.

6. If you have 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 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 Chapter 25). You might also want to temporarily modify pg_hba.conf to prevent ordinary users from connecting until you are sure the recovery was successful.

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 later) and then commence normal database operations.

9. 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. Not all of the requested files will be WAL segment files; you should also expect requests for files with a suffix of .backup or .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_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 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 23.3.4 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.

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.

23.3.4. Timelines

Because Coordinator and Datanode of Postgres-XC are essentially PostgreSQLserver, you can apply timelines for each Coordinator and Datanode manually.

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.

23.3.5. Tips and Examples

Some tips for configuring continuous archiving are given here.

archive_command = 'test ! -f /var/lib/pgsql/backup_in_progress || cp -i %p /var/lib/pgsql/archive/%f < /dev/null'

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/

archive_command = 'pg_compresslog %p - | gzip > /var/lib/pgsql/archive/%f'

restore_command = 'gunzip < /mnt/server/archivedir/%f | pg_decompresslog - %p'

archive_command = 'local_backup_script.sh'

23.3.6. 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. This will mean that any new inserts will be ignored by the index, updated rows will apparently disappear and deleted rows will still retain pointers. In other words, if you modify a table with a hash index on it then you will get incorrect query results on a standby server. When recovery completes it is recommended that you 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 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 28 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.