PostgreSQL 8.0.0beta3 Documentation | ||||
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At all times, PostgreSQL maintains a write ahead log (WAL) that shows details of 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 filesystem-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 notably more complex to administer than either of the previous approaches, but it has some significant benefits to offer:
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 filesystem 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 had simply by continuing to archive the WAL files. This is particularly valuable for large databases, where making a full backup may take an unreasonable amount of time.
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's 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 filesystem-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 is 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 an on-line backup, 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 the server). 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, or write them onto a tape drive, 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 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 %p /mnt/server/archivedir/%f'
which will copy archivable WAL segments to the directory /mnt/server/archivedir.
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 it may be overwritten with new data very soon thereafter. However, a nonzero status tells PostgreSQL that the file was not archived; it will try again periodically until it succeeds.
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. It is okay if the archiving process falls a little behind (or even a lot behind, if you don't mind the pg_xlog/ directory filling up with not-yet-archived segment files).
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. The combination of the archived WAL segments and the saved current segment will then 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 archive_command by requiring it to keep track of successively archived, but different, copies of the same WAL file. The archive_command is only invoked on finished WAL segments that will not change anymore; and except in the case of retrying a failure, it will be called only once for any given file name.
In writing your archive command, you should assume that the filenames 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).
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.) 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 filesystem-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();
If this returns successfully, you're done.
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 don't 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 at or after the starting time of the 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 part of
this file name stands for an exact position within the WAL file, and can
ordinarily be ignored.) Once you have safely archived the backup dump
file, you can delete all archived WAL segments with names numerically
preceding this one. 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 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 full dump, your interval between full dumps would 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 segments, and that could take awhile if it's been a long time since the full dump.
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 postmaster is
stopped. In this case, obviously you can't 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's generally better to follow the on-line backup procedure above.
Okay, the worst has happened and you need to recover from your backup. Here is the procedure:
Stop the postmaster, if it's running, and clean out all existing files under the cluster data directory and under the root directories of any tablespaces you are using. (If there are recent, unarchived WAL segment files in pg_xlog/ that you want to use during restore, move these aside instead of removing them.)
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 aside in step 1, copy them into pg_xlog/. (It's best to copy them, not move them back in, so that you still have the unmodified files if the worst happens and you have to start over.)
Create a recovery command file recovery.conf in the cluster data directory, as discussed below. 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 postmaster. The postmaster will go into recovery mode and proceed to read through the archived WAL files it needs. Upon completion of the recovery process, the postmaster 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 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 basename 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 current 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 either by date/time or completion of a specific transaction ID. The stop specification can be inclusive or exclusive. As of this writing only the date/time option is very usable, since there are no tools to help you identify which transaction ID to use. Keep in mind that while transaction IDs are asigned sequentially at transaction start, transactions can complete in a different numeric order.
Note that the stop point must be after the ending time of the backup
(ie, 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.)
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 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 on-line backup technique. These will probably be fixed in future releases:
Operations on non-btree indexes (hash, R-tree, and GiST indexes) are not presently WAL-logged, so replay will not update these index types. The recommended workaround, if you use any non-btree indexes, is to manually REINDEX each such index after completing a recovery operation.
It should also be noted that the present WAL format is extremely bulky since it includes many disk page snapshots. This is appropriate for crash recovery purposes, since we may need to fix partially-written disk pages. It is not necessary to store so many page copies for PITR operations, however. An area for future development is to compress archived WAL data by removing unnecessary page copies.