1 <!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.82 2006/08/06 03:53:43 tgl Exp $ -->
4 <title>Backup and Restore</title>
6 <indexterm zone="backup"><primary>backup</></>
9 As with everything that contains valuable data, <productname>PostgreSQL</>
10 databases should be backed up regularly. While the procedure is
11 essentially simple, it is important to have a basic understanding of
12 the underlying techniques and assumptions.
16 There are three fundamentally different approaches to backing up
17 <productname>PostgreSQL</> data:
19 <listitem><para><acronym>SQL</> dump</para></listitem>
20 <listitem><para>File system level backup</para></listitem>
21 <listitem><para>Continuous Archiving</para></listitem>
23 Each has its own strengths and weaknesses.
26 <sect1 id="backup-dump">
27 <title><acronym>SQL</> Dump</title>
30 The idea behind the SQL-dump method is to generate a text file with SQL
31 commands that, when fed back to the server, will recreate the
32 database in the same state as it was at the time of the dump.
33 <productname>PostgreSQL</> provides the utility program
34 <xref linkend="app-pgdump"> for this purpose. The basic usage of this
37 pg_dump <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">outfile</replaceable>
39 As you see, <application>pg_dump</> writes its results to the
40 standard output. We will see below how this can be useful.
44 <application>pg_dump</> is a regular <productname>PostgreSQL</>
45 client application (albeit a particularly clever one). This means
46 that you can do this backup procedure from any remote host that has
47 access to the database. But remember that <application>pg_dump</>
48 does not operate with special permissions. In particular, it must
49 have read access to all tables that you want to back up, so in
50 practice you almost always have to run it as a database superuser.
54 To specify which database server <application>pg_dump</> should
55 contact, use the command line options <option>-h
56 <replaceable>host</></> and <option>-p <replaceable>port</></>. The
57 default host is the local host or whatever your
58 <envar>PGHOST</envar> environment variable specifies. Similarly,
59 the default port is indicated by the <envar>PGPORT</envar>
60 environment variable or, failing that, by the compiled-in default.
61 (Conveniently, the server will normally have the same compiled-in
66 As any other <productname>PostgreSQL</> client application,
67 <application>pg_dump</> will by default connect with the database
68 user name that is equal to the current operating system user name. To override
69 this, either specify the <option>-U</option> option or set the
70 environment variable <envar>PGUSER</envar>. Remember that
71 <application>pg_dump</> connections are subject to the normal
72 client authentication mechanisms (which are described in <xref
73 linkend="client-authentication">).
77 Dumps created by <application>pg_dump</> are internally consistent,
78 that is, updates to the database while <application>pg_dump</> is
79 running will not be in the dump. <application>pg_dump</> does not
80 block other operations on the database while it is working.
81 (Exceptions are those operations that need to operate with an
82 exclusive lock, such as <command>VACUUM FULL</command>.)
87 When your database schema relies on OIDs (for instance as foreign
88 keys) you must instruct <application>pg_dump</> to dump the OIDs
89 as well. To do this, use the <option>-o</option> command line
94 <sect2 id="backup-dump-restore">
95 <title>Restoring the dump</title>
98 The text files created by <application>pg_dump</> are intended to
99 be read in by the <application>psql</application> program. The
100 general command form to restore a dump is
102 psql <replaceable class="parameter">dbname</replaceable> < <replaceable class="parameter">infile</replaceable>
104 where <replaceable class="parameter">infile</replaceable> is what
105 you used as <replaceable class="parameter">outfile</replaceable>
106 for the <application>pg_dump</> command. The database <replaceable
107 class="parameter">dbname</replaceable> will not be created by this
108 command, you must create it yourself from <literal>template0</> before executing
109 <application>psql</> (e.g., with <literal>createdb -T template0
110 <replaceable class="parameter">dbname</></literal>).
111 <application>psql</> supports options similar to <application>pg_dump</>
112 for controlling the database server location and the user name. See
113 <xref linkend="app-psql">'s reference page for more information.
117 Not only must the target database already exist before starting to
118 run the restore, but so must all the users who own objects in the
119 dumped database or were granted permissions on the objects. If they
120 do not, then the restore will fail to recreate the objects with the
121 original ownership and/or permissions. (Sometimes this is what you want,
122 but usually it is not.)
126 Once restored, it is wise to run <xref linkend="sql-analyze"
127 endterm="sql-analyze-title"> on each database so the optimizer has
128 useful statistics. An easy way to do this is to run
129 <command>vacuumdb -a -z</> to
130 <command>VACUUM ANALYZE</> all databases; this is equivalent to
131 running <command>VACUUM ANALYZE</command> manually.
135 The ability of <application>pg_dump</> and <application>psql</> to
136 write to or read from pipes makes it possible to dump a database
137 directly from one server to another; for example:
139 pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>host2</> <replaceable>dbname</>
145 The dumps produced by <application>pg_dump</> are relative to
146 <literal>template0</>. This means that any languages, procedures,
147 etc. added to <literal>template1</> will also be dumped by
148 <application>pg_dump</>. As a result, when restoring, if you are
149 using a customized <literal>template1</>, you must create the
150 empty database from <literal>template0</>, as in the example
156 For advice on how to load large amounts of data into
157 <productname>PostgreSQL</productname> efficiently, refer to <xref
162 <sect2 id="backup-dump-all">
163 <title>Using <application>pg_dumpall</></title>
166 The above mechanism is cumbersome and inappropriate when backing
167 up an entire database cluster. For this reason the <xref
168 linkend="app-pg-dumpall"> program is provided.
169 <application>pg_dumpall</> backs up each database in a given
170 cluster, and also preserves cluster-wide data such as users and
171 groups. The basic usage of this command is:
173 pg_dumpall > <replaceable>outfile</>
175 The resulting dump can be restored with <application>psql</>:
177 psql -f <replaceable class="parameter">infile</replaceable> postgres
179 (Actually, you can specify any existing database name to start from,
180 but if you are reloading in an empty cluster then <literal>postgres</>
181 should generally be used.) It is always necessary to have
182 database superuser access when restoring a <application>pg_dumpall</>
183 dump, as that is required to restore the user and group information.
187 <sect2 id="backup-dump-large">
188 <title>Handling large databases</title>
191 Since <productname>PostgreSQL</productname> allows tables larger
192 than the maximum file size on your system, it can be problematic
193 to dump such a table to a file, since the resulting file will likely
194 be larger than the maximum size allowed by your system. Since
195 <application>pg_dump</> can write to the standard output, you can
196 just use standard Unix tools to work around this possible problem.
200 <title>Use compressed dumps.</title>
202 You can use your favorite compression program, for example
203 <application>gzip</application>.
206 pg_dump <replaceable class="parameter">dbname</replaceable> | gzip > <replaceable class="parameter">filename</replaceable>.gz
212 createdb <replaceable class="parameter">dbname</replaceable>
213 gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
219 cat <replaceable class="parameter">filename</replaceable>.gz | gunzip | psql <replaceable class="parameter">dbname</replaceable>
225 <title>Use <command>split</>.</title>
227 The <command>split</command> command
228 allows you to split the output into pieces that are
229 acceptable in size to the underlying file system. For example, to
230 make chunks of 1 megabyte:
233 pg_dump <replaceable class="parameter">dbname</replaceable> | split -b 1m - <replaceable class="parameter">filename</replaceable>
239 createdb <replaceable class="parameter">dbname</replaceable>
240 cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
246 <title>Use the custom dump format.</title>
248 If <productname>PostgreSQL</productname> was built on a system with the
249 <application>zlib</> compression library installed, the custom dump
250 format will compress data as it writes it to the output file. This will
251 produce dump file sizes similar to using <command>gzip</command>, but it
252 has the added advantage that tables can be restored selectively. The
253 following command dumps a database using the custom dump format:
256 pg_dump -Fc <replaceable class="parameter">dbname</replaceable> > <replaceable class="parameter">filename</replaceable>
259 A custom-format dump is not a script for <application>psql</>, but
260 instead must be restored with <application>pg_restore</>.
261 See the <xref linkend="app-pgdump"> and <xref
262 linkend="app-pgrestore"> reference pages for details.
269 <sect1 id="backup-file">
270 <title>File system level backup</title>
273 An alternative backup strategy is to directly copy the files that
274 <productname>PostgreSQL</> uses to store the data in the database. In
275 <xref linkend="creating-cluster"> it is explained where these files
276 are located, but you have probably found them already if you are
277 interested in this method. You can use whatever method you prefer
278 for doing usual file system backups, for example
281 tar -cf backup.tar /usr/local/pgsql/data
286 There are two restrictions, however, which make this method
287 impractical, or at least inferior to the <application>pg_dump</>
293 The database server <emphasis>must</> be shut down in order to
294 get a usable backup. Half-way measures such as disallowing all
295 connections will <emphasis>not</emphasis> work
296 (mainly because <command>tar</command> and similar tools do not take an
297 atomic snapshot of the state of the file system at a point in
298 time). Information about stopping the server can be found in
299 <xref linkend="server-shutdown">. Needless to say that you
300 also need to shut down the server before restoring the data.
306 If you have dug into the details of the file system layout of the
307 database, you may be tempted to try to back up or restore only certain
308 individual tables or databases from their respective files or
309 directories. This will <emphasis>not</> work because the
310 information contained in these files contains only half the
311 truth. The other half is in the commit log files
312 <filename>pg_clog/*</filename>, which contain the commit status of
313 all transactions. A table file is only usable with this
314 information. Of course it is also impossible to restore only a
315 table and the associated <filename>pg_clog</filename> data
316 because that would render all other tables in the database
317 cluster useless. So file system backups only work for complete
318 restoration of an entire database cluster.
325 An alternative file-system backup approach is to make a
326 <quote>consistent snapshot</quote> of the data directory, if the
327 file system supports that functionality (and you are willing to
328 trust that it is implemented correctly). The typical procedure is
329 to make a <quote>frozen snapshot</> of the volume containing the
330 database, then copy the whole data directory (not just parts, see
331 above) from the snapshot to a backup device, then release the frozen
332 snapshot. This will work even while the database server is running.
333 However, a backup created in this way saves
334 the database files in a state where the database server was not
335 properly shut down; therefore, when you start the database server
336 on the backed-up data, it will think the server had crashed
337 and replay the WAL log. This is not a problem, just be aware of
338 it (and be sure to include the WAL files in your backup).
342 If your database is spread across multiple file systems, there may not
343 be any way to obtain exactly-simultaneous frozen snapshots of all
344 the volumes. For example, if your data files and WAL log are on different
345 disks, or if tablespaces are on different file systems, it might
346 not be possible to use snapshot backup because the snapshots must be
348 Read your file system documentation very carefully before trusting
349 to the consistent-snapshot technique in such situations. The safest
350 approach is to shut down the database server for long enough to
351 establish all the frozen snapshots.
355 Another option is to use <application>rsync</> to perform a file
356 system backup. This is done by first running <application>rsync</>
357 while the database server is running, then shutting down the database
358 server just long enough to do a second <application>rsync</>. The
359 second <application>rsync</> will be much quicker than the first,
360 because it has relatively little data to transfer, and the end result
361 will be consistent because the server was down. This method
362 allows a file system backup to be performed with minimal downtime.
366 Note that a file system backup will not necessarily be
367 smaller than an SQL dump. On the contrary, it will most likely be
368 larger. (<application>pg_dump</application> does not need to dump
369 the contents of indexes for example, just the commands to recreate
374 <sect1 id="continuous-archiving">
375 <title>Continuous Archiving and Point-In-Time Recovery (PITR)</title>
377 <indexterm zone="backup">
378 <primary>continuous archiving</primary>
381 <indexterm zone="backup">
382 <primary>point-in-time recovery</primary>
385 <indexterm zone="backup">
386 <primary>PITR</primary>
390 At all times, <productname>PostgreSQL</> maintains a
391 <firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
392 subdirectory of the cluster's data directory. The log describes
393 every change made to the database's data files. This log exists
394 primarily for crash-safety purposes: if the system crashes, the
395 database can be restored to consistency by <quote>replaying</> the
396 log entries made since the last checkpoint. However, the existence
397 of the log makes it possible to use a third strategy for backing up
398 databases: we can combine a file-system-level backup with backup of
399 the WAL files. If recovery is needed, we restore the backup and
400 then replay from the backed-up WAL files to bring the backup up to
401 current time. This approach is more complex to administer than
402 either of the previous approaches, but it has some significant
407 We do not need a perfectly consistent backup as the starting point.
408 Any internal inconsistency in the backup will be corrected by log
409 replay (this is not significantly different from what happens during
410 crash recovery). So we don't need file system snapshot capability,
411 just <application>tar</> or a similar archiving tool.
416 Since we can string together an indefinitely long sequence of WAL files
417 for replay, continuous backup can be achieved simply by continuing to archive
418 the WAL files. This is particularly valuable for large databases, where
419 it may not be convenient to take a full backup frequently.
424 There is nothing that says we have to replay the WAL entries all the
425 way to the end. We could stop the replay at any point and have a
426 consistent snapshot of the database as it was at that time. Thus,
427 this technique supports <firstterm>point-in-time recovery</>: it is
428 possible to restore the database to its state at any time since your base
434 If we continuously feed the series of WAL files to another
435 machine that has been loaded with the same base backup file, we
436 have a <quote>hot standby</> system: at any point we can bring up
437 the second machine and it will have a nearly-current copy of the
445 As with the plain file-system-backup technique, this method can only
446 support restoration of an entire database cluster, not a subset.
447 Also, it requires a lot of archival storage: the base backup may be bulky,
448 and a busy system will generate many megabytes of WAL traffic that
449 have to be archived. Still, it is the preferred backup technique in
450 many situations where high reliability is needed.
454 To recover successfully using continuous archiving (also called "online
455 backup" by many database vendors), you need a continuous
456 sequence of archived WAL files that extends back at least as far as the
457 start time of your backup. So to get started, you should set up and test
458 your procedure for archiving WAL files <emphasis>before</> you take your
459 first base backup. Accordingly, we first discuss the mechanics of
463 <sect2 id="backup-archiving-wal">
464 <title>Setting up WAL archiving</title>
467 In an abstract sense, a running <productname>PostgreSQL</> system
468 produces an indefinitely long sequence of WAL records. The system
469 physically divides this sequence into WAL <firstterm>segment
470 files</>, which are normally 16MB apiece (although the size can be
471 altered when building <productname>PostgreSQL</>). The segment
472 files are given numeric names that reflect their position in the
473 abstract WAL sequence. When not using WAL archiving, the system
474 normally creates just a few segment files and then
475 <quote>recycles</> them by renaming no-longer-needed segment files
476 to higher segment numbers. It's assumed that a segment file whose
477 contents precede the checkpoint-before-last is no longer of
478 interest and can be recycled.
482 When archiving WAL data, we want to capture the contents of each segment
483 file once it is filled, and save that data somewhere before the segment
484 file is recycled for reuse. Depending on the application and the
485 available hardware, there could be many different ways of <quote>saving
486 the data somewhere</>: we could copy the segment files to an NFS-mounted
487 directory on another machine, write them onto a tape drive (ensuring that
488 you have a way of restoring the file with its original file name), or batch
489 them together and burn them onto CDs, or something else entirely. To
490 provide the database administrator with as much flexibility as possible,
491 <productname>PostgreSQL</> tries not to make any assumptions about how
492 the archiving will be done. Instead, <productname>PostgreSQL</> lets
493 the administrator specify a shell command to be executed to copy a
494 completed segment file to wherever it needs to go. The command could be
495 as simple as a <application>cp</>, or it could invoke a complex shell
496 script — it's all up to you.
500 The shell command to use is specified by the <xref
501 linkend="guc-archive-command"> configuration parameter, which in practice
502 will always be placed in the <filename>postgresql.conf</filename> file.
504 any <literal>%p</> is replaced by the absolute path of the file to
505 archive, while any <literal>%f</> is replaced by the file name only.
506 Write <literal>%%</> if you need to embed an actual <literal>%</>
507 character in the command. The simplest useful command is something
510 archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
512 which will copy archivable WAL segments to the directory
513 <filename>/mnt/server/archivedir</>. (This is an example, not a
514 recommendation, and may not work on all platforms.)
518 The archive command will be executed under the ownership of the same
519 user that the <productname>PostgreSQL</> server is running as. Since
520 the series of WAL files being archived contains effectively everything
521 in your database, you will want to be sure that the archived data is
522 protected from prying eyes; for example, archive into a directory that
523 does not have group or world read access.
527 It is important that the archive command return zero exit status if and
528 only if it succeeded. Upon getting a zero result,
529 <productname>PostgreSQL</> will assume that the WAL segment file has been
530 successfully archived, and will remove or recycle it.
531 However, a nonzero status tells
532 <productname>PostgreSQL</> that the file was not archived; it will try
533 again periodically until it succeeds.
537 The archive command should generally be designed to refuse to overwrite
538 any pre-existing archive file. This is an important safety feature to
539 preserve the integrity of your archive in case of administrator error
540 (such as sending the output of two different servers to the same archive
542 It is advisable to test your proposed archive command to ensure that it
543 indeed does not overwrite an existing file, <emphasis>and that it returns
544 nonzero status in this case</>. We have found that <literal>cp -i</> does
545 this correctly on some platforms but not others. If the chosen command
546 does not itself handle this case correctly, you should add a command
547 to test for pre-existence of the archive file. For example, something
550 archive_command = 'test ! -f .../%f && cp %p .../%f'
552 works correctly on most Unix variants.
556 While designing your archiving setup, consider what will happen if
557 the archive command fails repeatedly because some aspect requires
558 operator intervention or the archive runs out of space. For example, this
559 could occur if you write to tape without an autochanger; when the tape
560 fills, nothing further can be archived until the tape is swapped.
561 You should ensure that any error condition or request to a human operator
562 is reported appropriately so that the situation can be
563 resolved relatively quickly. The <filename>pg_xlog/</> directory will
564 continue to fill with WAL segment files until the situation is resolved.
568 The speed of the archiving command is not important, so long as it can keep up
569 with the average rate at which your server generates WAL data. Normal
570 operation continues even if the archiving process falls a little behind.
571 If archiving falls significantly behind, this will increase the amount of
572 data that would be lost in the event of a disaster. It will also mean that
573 the <filename>pg_xlog/</> directory will contain large numbers of
574 not-yet-archived segment files, which could eventually exceed available
575 disk space. You are advised to monitor the archiving process to ensure that
576 it is working as you intend.
580 If you are concerned about being able to recover right up to the
581 current instant, you may want to take additional steps to ensure that
582 the current, partially-filled WAL segment is also copied someplace.
583 This is particularly important if your server generates only little WAL
584 traffic (or has slack periods where it does so), since it could take a
585 long time before a WAL segment file is completely filled and ready to
586 archive. One possible way to handle this is to set up a
587 <application>cron</> job that periodically (once a minute, perhaps)
588 identifies the current WAL segment file and saves it someplace safe.
589 Then the combination of the archived WAL segments and the saved current
590 segment will be enough to ensure you can always restore to within a
591 minute of current time. This behavior is not presently built into
592 <productname>PostgreSQL</> because we did not want to complicate the
593 definition of the <xref linkend="guc-archive-command"> by requiring it
594 to keep track of successively archived, but different, copies of the
595 same WAL file. The <xref linkend="guc-archive-command"> is only
596 invoked on completed WAL segments. Except in the case of retrying a
597 failure, it will be called only once for any given file name.
601 Another way to limit your exposure to data loss is to call
602 <function>pg_switch_xlog()</> periodically, such as once a minute.
603 This function forces the current WAL segment file to be completed
604 and made available to the archiving command. This approach does
605 not work well for extremely short update intervals, however, since
606 copying a new 16MB segment file every few seconds is expensive.
610 In writing your archive command, you should assume that the file names to
611 be archived may be up to 64 characters long and may contain any
612 combination of ASCII letters, digits, and dots. It is not necessary to
613 remember the original full path (<literal>%p</>) but it is necessary to
614 remember the file name (<literal>%f</>).
618 Note that although WAL archiving will allow you to restore any
619 modifications made to the data in your <productname>PostgreSQL</> database
620 it will not restore changes made to configuration files (that is,
621 <filename>postgresql.conf</>, <filename>pg_hba.conf</> and
622 <filename>pg_ident.conf</>), since those are edited manually rather
623 than through SQL operations.
624 You may wish to keep the configuration files in a location that will
625 be backed up by your regular file system backup procedures. See
626 <xref linkend="runtime-config-file-locations"> for how to relocate the
631 <sect2 id="backup-base-backup">
632 <title>Making a Base Backup</title>
635 The procedure for making a base backup is relatively simple:
639 Ensure that WAL archiving is enabled and working.
644 Connect to the database as a superuser, and issue the command
646 SELECT pg_start_backup('label');
648 where <literal>label</> is any string you want to use to uniquely
649 identify this backup operation. (One good practice is to use the
650 full path where you intend to put the backup dump file.)
651 <function>pg_start_backup</> creates a <firstterm>backup label</> file,
652 called <filename>backup_label</>, in the cluster directory with
653 information about your backup.
657 It does not matter which database within the cluster you connect to to
658 issue this command. You can ignore the result returned by the function;
659 but if it reports an error, deal with that before proceeding.
664 Perform the backup, using any convenient file-system-backup tool
665 such as <application>tar</> or <application>cpio</>. It is neither
666 necessary nor desirable to stop normal operation of the database
672 Again connect to the database as a superuser, and issue the command
674 SELECT pg_stop_backup();
676 This should return successfully.
681 Once the WAL segment files used during the backup are archived as part
682 of normal database activity, you are done. The file identified by
683 <function>pg_stop_backup</>'s result is the last segment that needs
684 to be archived to complete the backup.
691 Some backup tools that you might wish to use emit warnings or errors
692 if the files they are trying to copy change while the copy proceeds.
693 This situation is normal, and not an error, when taking a base backup of
694 an active database; so you need to ensure that you can distinguish
695 complaints of this sort from real errors. For example, some versions
696 of <application>rsync</> return a separate exit code for <quote>vanished
697 source files</>, and you can write a driver script to accept this exit
698 code as a non-error case. Also,
699 some versions of GNU <application>tar</> consider it an error if a file
700 is changed while <application>tar</> is copying it. There does not seem
701 to be any very convenient way to distinguish this error from other types
702 of errors, other than manual inspection of <application>tar</>'s messages.
703 GNU <application>tar</> is therefore not the best tool for making base
708 It is not necessary to be very concerned about the amount of time elapsed
709 between <function>pg_start_backup</> and the start of the actual backup,
710 nor between the end of the backup and <function>pg_stop_backup</>; a
711 few minutes' delay won't hurt anything. You
712 must however be quite sure that these operations are carried out in
713 sequence and do not overlap.
717 Be certain that your backup dump includes all of the files underneath
718 the database cluster directory (e.g., <filename>/usr/local/pgsql/data</>).
719 If you are using tablespaces that do not reside underneath this directory,
720 be careful to include them as well (and be sure that your backup dump
721 archives symbolic links as links, otherwise the restore will mess up
726 You may, however, omit from the backup dump the files within the
727 <filename>pg_xlog/</> subdirectory of the cluster directory. This
728 slight complication is worthwhile because it reduces the risk
729 of mistakes when restoring. This is easy to arrange if
730 <filename>pg_xlog/</> is a symbolic link pointing to someplace outside
731 the cluster directory, which is a common setup anyway for performance
736 To make use of this backup, you will need to keep around all the WAL
737 segment files generated during and after the file system backup.
738 To aid you in doing this, the <function>pg_stop_backup</> function
739 creates a <firstterm>backup history file</> that is immediately
740 stored into the WAL archive area. This file is named after the first
741 WAL segment file that you need to have to make use of the backup.
742 For example, if the starting WAL file is
743 <literal>0000000100001234000055CD</> the backup history file will be
745 <literal>0000000100001234000055CD.007C9330.backup</>. (The second
746 number in the file name stands for an exact position within the WAL
747 file, and can ordinarily be ignored.) Once you have safely archived
748 the file system backup and the WAL segment files used during the
749 backup (as specified in the backup history file), all archived WAL
750 segments with names numerically less are no longer needed to recover
751 the file system backup and may be deleted. However, you should
752 consider keeping several backup sets to be absolutely certain that
753 you can recover your data.
757 The backup history file is just a small text file. It contains the
758 label string you gave to <function>pg_start_backup</>, as well as
759 the starting and ending times and WAL segments of the backup.
760 If you used the label to identify where the associated dump file is kept,
761 then the archived history file is enough to tell you which dump file to
762 restore, should you need to do so.
766 Since you have to keep around all the archived WAL files back to your
767 last base backup, the interval between base backups should usually be
768 chosen based on how much storage you want to expend on archived WAL
769 files. You should also consider how long you are prepared to spend
770 recovering, if recovery should be necessary — the system will have to
771 replay all those WAL segments, and that could take awhile if it has
772 been a long time since the last base backup.
776 It's also worth noting that the <function>pg_start_backup</> function
777 makes a file named <filename>backup_label</> in the database cluster
778 directory, which is then removed again by <function>pg_stop_backup</>.
779 This file will of course be archived as a part of your backup dump file.
780 The backup label file includes the label string you gave to
781 <function>pg_start_backup</>, as well as the time at which
782 <function>pg_start_backup</> was run, and the name of the starting WAL
783 file. In case of confusion it will
784 therefore be possible to look inside a backup dump file and determine
785 exactly which backup session the dump file came from.
789 It is also possible to make a backup dump while the server is
790 stopped. In this case, you obviously cannot use
791 <function>pg_start_backup</> or <function>pg_stop_backup</>, and
792 you will therefore be left to your own devices to keep track of which
793 backup dump is which and how far back the associated WAL files go.
794 It is generally better to follow the continuous archiving procedure above.
798 <sect2 id="backup-pitr-recovery">
799 <title>Recovering using a Continuous Archive Backup</title>
802 Okay, the worst has happened and you need to recover from your backup.
803 Here is the procedure:
807 Stop the server, if it's running.
812 If you have the space to do so,
813 copy the whole cluster data directory and any tablespaces to a temporary
814 location in case you need them later. Note that this precaution will
815 require that you have enough free space on your system to hold two
816 copies of your existing database. If you do not have enough space,
817 you need at the least to copy the contents of the <filename>pg_xlog</>
818 subdirectory of the cluster data directory, as it may contain logs which
819 were not archived before the system went down.
824 Clean out all existing files and subdirectories under the cluster data
825 directory and under the root directories of any tablespaces you are using.
830 Restore the database files from your backup dump. Be careful that they
831 are restored with the right ownership (the database system user, not
832 root!) and with the right permissions. If you are using tablespaces,
833 you may want to verify that the symbolic links in <filename>pg_tblspc/</>
834 were correctly restored.
839 Remove any files present in <filename>pg_xlog/</>; these came from the
840 backup dump and are therefore probably obsolete rather than current.
841 If you didn't archive <filename>pg_xlog/</> at all, then re-create it,
842 and be sure to re-create the subdirectory
843 <filename>pg_xlog/archive_status/</> as well.
848 If you had unarchived WAL segment files that you saved in step 2,
849 copy them into <filename>pg_xlog/</>. (It is best to copy them,
850 not move them, so that you still have the unmodified files if a
851 problem occurs and you have to start over.)
856 Create a recovery command file <filename>recovery.conf</> in the cluster
857 data directory (see <xref linkend="recovery-config-settings">). You may
858 also want to temporarily modify <filename>pg_hba.conf</> to prevent
859 ordinary users from connecting until you are sure the recovery has worked.
864 Start the server. The server will go into recovery mode and
865 proceed to read through the archived WAL files it needs. Upon completion
866 of the recovery process, the server will rename
867 <filename>recovery.conf</> to <filename>recovery.done</> (to prevent
868 accidentally re-entering recovery mode in case of a crash later) and then
869 commence normal database operations.
874 Inspect the contents of the database to ensure you have recovered to
875 where you want to be. If not, return to step 1. If all is well,
876 let in your users by restoring <filename>pg_hba.conf</> to normal.
883 The key part of all this is to set up a recovery command file that
884 describes how you want to recover and how far the recovery should
885 run. You can use <filename>recovery.conf.sample</> (normally
886 installed in the installation <filename>share/</> directory) as a
887 prototype. The one thing that you absolutely must specify in
888 <filename>recovery.conf</> is the <varname>restore_command</>,
889 which tells <productname>PostgreSQL</> how to get back archived
890 WAL file segments. Like the <varname>archive_command</>, this is
891 a shell command string. It may contain <literal>%f</>, which is
892 replaced by the name of the desired log file, and <literal>%p</>,
893 which is replaced by the absolute path to copy the log file to.
894 Write <literal>%%</> if you need to embed an actual <literal>%</>
895 character in the command. The simplest useful command is
898 restore_command = 'cp /mnt/server/archivedir/%f %p'
900 which will copy previously archived WAL segments from the directory
901 <filename>/mnt/server/archivedir</>. You could of course use something
902 much more complicated, perhaps even a shell script that requests the
903 operator to mount an appropriate tape.
907 It is important that the command return nonzero exit status on failure.
908 The command <emphasis>will</> be asked for log files that are not present
909 in the archive; it must return nonzero when so asked. This is not an
910 error condition. Be aware also that the base name of the <literal>%p</>
911 path will be different from <literal>%f</>; do not expect them to be
916 WAL segments that cannot be found in the archive will be sought in
917 <filename>pg_xlog/</>; this allows use of recent un-archived segments.
918 However segments that are available from the archive will be used in
919 preference to files in <filename>pg_xlog/</>. The system will not
920 overwrite the existing contents of <filename>pg_xlog/</> when retrieving
925 Normally, recovery will proceed through all available WAL segments,
926 thereby restoring the database to the current point in time (or as
927 close as we can get given the available WAL segments). But if you want
928 to recover to some previous point in time (say, right before the junior
929 DBA dropped your main transaction table), just specify the required
930 stopping point in <filename>recovery.conf</>. You can specify the stop
931 point, known as the <quote>recovery target</>, either by date/time or
932 by completion of a specific transaction ID. As of this writing only
933 the date/time option is very usable, since there are no tools to help
934 you identify with any accuracy which transaction ID to use.
939 The stop point must be after the ending time of the base backup (the
940 time of <function>pg_stop_backup</>). You cannot use a base backup
941 to recover to a time when that backup was still going on. (To
942 recover to such a time, you must go back to your previous base backup
943 and roll forward from there.)
947 <sect3 id="recovery-config-settings" xreflabel="Recovery Settings">
948 <title>Recovery Settings</title>
951 These settings can only be made in the <filename>recovery.conf</>
952 file, and apply only for the duration of the recovery. They must be
953 reset for any subsequent recovery you wish to perform. They cannot be
954 changed once recovery has begun.
959 <varlistentry id="restore-command" xreflabel="restore_command">
960 <term><varname>restore_command</varname> (<type>string</type>)</term>
963 The shell command to execute to retrieve an archived segment of
964 the WAL file series. This parameter is required.
965 Any <literal>%f</> in the string is
966 replaced by the name of the file to retrieve from the archive,
967 and any <literal>%p</> is replaced by the absolute path to copy
969 Write <literal>%%</> to embed an actual <literal>%</> character
973 It is important for the command to return a zero exit status if and
974 only if it succeeds. The command <emphasis>will</> be asked for file
975 names that are not present in the archive; it must return nonzero
976 when so asked. Examples:
978 restore_command = 'cp /mnt/server/archivedir/%f "%p"'
979 restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
985 <varlistentry id="recovery-target-time" xreflabel="recovery_target_time">
986 <term><varname>recovery_target_time</varname>
987 (<type>timestamp</type>)
991 This parameter specifies the time stamp up to which recovery
993 At most one of <varname>recovery_target_time</> and
994 <xref linkend="recovery-target-xid"> can be specified.
995 The default is to recover to the end of the WAL log.
996 The precise stopping point is also influenced by
997 <xref linkend="recovery-target-inclusive">.
1002 <varlistentry id="recovery-target-xid" xreflabel="recovery_target_xid">
1003 <term><varname>recovery_target_xid</varname> (<type>string</type>)</term>
1006 This parameter specifies the transaction ID up to which recovery
1007 will proceed. Keep in mind
1008 that while transaction IDs are assigned sequentially at transaction
1009 start, transactions can complete in a different numeric order.
1010 The transactions that will be recovered are those that committed
1011 before (and optionally including) the specified one.
1012 At most one of <varname>recovery_target_xid</> and
1013 <xref linkend="recovery-target-time"> can be specified.
1014 The default is to recover to the end of the WAL log.
1015 The precise stopping point is also influenced by
1016 <xref linkend="recovery-target-inclusive">.
1021 <varlistentry id="recovery-target-inclusive"
1022 xreflabel="recovery_target_inclusive">
1023 <term><varname>recovery_target_inclusive</varname>
1024 (<type>boolean</type>)
1028 Specifies whether we stop just after the specified recovery target
1029 (<literal>true</literal>), or just before the recovery target
1030 (<literal>false</literal>).
1031 Applies to both <xref linkend="recovery-target-time">
1032 and <xref linkend="recovery-target-xid">, whichever one is
1033 specified for this recovery. This indicates whether transactions
1034 having exactly the target commit time or ID, respectively, will
1035 be included in the recovery. Default is <literal>true</>.
1040 <varlistentry id="recovery-target-timeline"
1041 xreflabel="recovery_target_timeline">
1042 <term><varname>recovery_target_timeline</varname>
1043 (<type>string</type>)
1047 Specifies recovering into a particular timeline. The default is
1048 to recover along the same timeline that was current when the
1049 base backup was taken. You would only need to set this parameter
1050 in complex re-recovery situations, where you need to return to
1051 a state that itself was reached after a point-in-time recovery.
1052 See <xref linkend="backup-timelines"> for discussion.
1063 <sect2 id="backup-timelines">
1064 <title>Timelines</title>
1066 <indexterm zone="backup">
1067 <primary>timelines</primary>
1071 The ability to restore the database to a previous point in time creates
1072 some complexities that are akin to science-fiction stories about time
1073 travel and parallel universes. In the original history of the database,
1074 perhaps you dropped a critical table at 5:15PM on Tuesday evening.
1075 Unfazed, you get out your backup, restore to the point-in-time 5:14PM
1076 Tuesday evening, and are up and running. In <emphasis>this</> history of
1077 the database universe, you never dropped the table at all. But suppose
1078 you later realize this wasn't such a great idea after all, and would like
1079 to return to some later point in the original history. You won't be able
1080 to if, while your database was up-and-running, it overwrote some of the
1081 sequence of WAL segment files that led up to the time you now wish you
1082 could get back to. So you really want to distinguish the series of
1083 WAL records generated after you've done a point-in-time recovery from
1084 those that were generated in the original database history.
1088 To deal with these problems, <productname>PostgreSQL</> has a notion
1089 of <firstterm>timelines</>. Each time you recover to a point-in-time
1090 earlier than the end of the WAL sequence, a new timeline is created
1091 to identify the series of WAL records generated after that recovery.
1092 (If recovery proceeds all the way to the end of WAL, however, we do not
1093 start a new timeline: we just extend the existing one.) The timeline
1094 ID number is part of WAL segment file names, and so a new timeline does
1095 not overwrite the WAL data generated by previous timelines. It is
1096 in fact possible to archive many different timelines. While that might
1097 seem like a useless feature, it's often a lifesaver. Consider the
1098 situation where you aren't quite sure what point-in-time to recover to,
1099 and so have to do several point-in-time recoveries by trial and error
1100 until you find the best place to branch off from the old history. Without
1101 timelines this process would soon generate an unmanageable mess. With
1102 timelines, you can recover to <emphasis>any</> prior state, including
1103 states in timeline branches that you later abandoned.
1107 Each time a new timeline is created, <productname>PostgreSQL</> creates
1108 a <quote>timeline history</> file that shows which timeline it branched
1109 off from and when. These history files are necessary to allow the system
1110 to pick the right WAL segment files when recovering from an archive that
1111 contains multiple timelines. Therefore, they are archived into the WAL
1112 archive area just like WAL segment files. The history files are just
1113 small text files, so it's cheap and appropriate to keep them around
1114 indefinitely (unlike the segment files which are large). You can, if
1115 you like, add comments to a history file to make your own notes about
1116 how and why this particular timeline came to be. Such comments will be
1117 especially valuable when you have a thicket of different timelines as
1118 a result of experimentation.
1122 The default behavior of recovery is to recover along the same timeline
1123 that was current when the base backup was taken. If you want to recover
1124 into some child timeline (that is, you want to return to some state that
1125 was itself generated after a recovery attempt), you need to specify the
1126 target timeline ID in <filename>recovery.conf</>. You cannot recover into
1127 timelines that branched off earlier than the base backup.
1131 <sect2 id="continuous-archiving-caveats">
1132 <title>Caveats</title>
1135 At this writing, there are several limitations of the continuous archiving
1136 technique. These will probably be fixed in future releases:
1141 Operations on hash indexes are
1142 not presently WAL-logged, so replay will not update these indexes.
1143 The recommended workaround is to manually <command>REINDEX</> each
1144 such index after completing a recovery operation.
1150 If a <command>CREATE DATABASE</> command is executed while a base
1151 backup is being taken, and then the template database that the
1152 <command>CREATE DATABASE</> copied is modified while the base backup
1153 is still in progress, it is possible that recovery will cause those
1154 modifications to be propagated into the created database as well.
1155 This is of course undesirable. To avoid this risk, it is best not to
1156 modify any template databases while taking a base backup.
1162 <command>CREATE TABLESPACE</> commands are WAL-logged with the literal
1163 absolute path, and will therefore be replayed as tablespace creations
1164 with the same absolute path. This might be undesirable if the log is
1165 being replayed on a different machine. It can be dangerous even if
1166 the log is being replayed on the same machine, but into a new data
1167 directory: the replay will still overwrite the contents of the original
1168 tablespace. To avoid potential gotchas of this sort, the best practice
1169 is to take a new base backup after creating or dropping tablespaces.
1176 It should also be noted that the default <acronym>WAL</acronym>
1177 format is fairly bulky since it includes many disk page snapshots.
1178 These page snapshots are designed to support crash recovery,
1179 since we may need to fix partially-written disk pages. Depending
1180 on your system hardware and software, the risk of partial writes may
1181 be small enough to ignore, in which case you can significantly reduce
1182 the total volume of archived logs by turning off page snapshots
1183 using the <xref linkend="guc-full-page-writes"> parameter.
1184 (Read the notes and warnings in
1185 <xref linkend="wal"> before you do so.)
1186 Turning off page snapshots does not prevent use of the logs for PITR
1188 An area for future development is to compress archived WAL data by
1189 removing unnecessary page copies even when <varname>full_page_writes</>
1190 is on. In the meantime, administrators
1191 may wish to reduce the number of page snapshots included in WAL by
1192 increasing the checkpoint interval parameters as much as feasible.
1197 <sect1 id="migration">
1198 <title>Migration Between Releases</title>
1200 <indexterm zone="migration">
1201 <primary>upgrading</primary>
1204 <indexterm zone="migration">
1205 <primary>version</primary>
1206 <secondary>compatibility</secondary>
1210 This section discusses how to migrate your database data from one
1211 <productname>PostgreSQL</> release to a newer one.
1212 The software installation procedure <foreignphrase>per se</> is not the
1213 subject of this section; those details are in <xref linkend="installation">.
1217 As a general rule, the internal data storage format is subject to
1218 change between major releases of <productname>PostgreSQL</> (where
1219 the number after the first dot changes). This does not apply to
1220 different minor releases under the same major release (where the
1221 number after the second dot changes); these always have compatible
1222 storage formats. For example, releases 7.2.1, 7.3.2, and 7.4 are
1223 not compatible, whereas 7.2.1 and 7.2.2 are. When you update
1224 between compatible versions, you can simply replace the executables
1225 and reuse the data directory on disk. Otherwise you need to back
1226 up your data and restore it on the new server. This has to be done
1227 using <application>pg_dump</>; file system level backup methods
1228 obviously won't work. There are checks in place that prevent you
1229 from using a data directory with an incompatible version of
1230 <productname>PostgreSQL</productname>, so no great harm can be done by
1231 trying to start the wrong server version on a data directory.
1235 It is recommended that you use the <application>pg_dump</> and
1236 <application>pg_dumpall</> programs from the newer version of
1237 <productname>PostgreSQL</>, to take advantage of any enhancements
1238 that may have been made in these programs. Current releases of the
1239 dump programs can read data from any server version back to 7.0.
1243 The least downtime can be achieved by installing the new server in
1244 a different directory and running both the old and the new servers
1245 in parallel, on different ports. Then you can use something like
1248 pg_dumpall -p 5432 | psql -d postgres -p 6543
1251 to transfer your data. Or use an intermediate file if you want.
1252 Then you can shut down the old server and start the new server at
1253 the port the old one was running at. You should make sure that the
1254 old database is not updated after you run <application>pg_dumpall</>,
1255 otherwise you will obviously lose that data. See <xref
1256 linkend="client-authentication"> for information on how to prohibit
1261 In practice you probably want to test your client
1262 applications on the new setup before switching over completely.
1263 This is another reason for setting up concurrent installations
1264 of old and new versions.
1268 If you cannot or do not want to run two servers in parallel you can
1269 do the backup step before installing the new version, bring down
1270 the server, move the old version out of the way, install the new
1271 version, start the new server, restore the data. For example:
1274 pg_dumpall > backup
1276 mv /usr/local/pgsql /usr/local/pgsql.old
1277 cd ~/postgresql-&version;
1279 initdb -D /usr/local/pgsql/data
1280 postgres -D /usr/local/pgsql/data
1281 psql -f backup postgres
1284 See <xref linkend="runtime"> about ways to start and stop the
1285 server and other details. The installation instructions will advise
1286 you of strategic places to perform these steps.
1291 When you <quote>move the old installation out of the way</quote>
1292 it may no longer be perfectly usable. Some of the executable programs
1293 contain absolute paths to various installed programs and data files.
1294 This is usually not a big problem but if you plan on using two
1295 installations in parallel for a while you should assign them
1296 different installation directories at build time. (This problem
1297 is rectified in <productname>PostgreSQL</> 8.0 and later, but you
1298 need to be wary of moving older installations.)
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