-<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.110 2007/12/15 15:41:02 adunstan Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.111 2007/12/17 04:30:05 tgl Exp $ -->
<chapter id="backup">
<title>Backup and Restore</title>
<para>
As with everything that contains valuable data, <productname>PostgreSQL</>
databases should be backed up regularly. While the procedure is
- essentially simple, it is important to have a basic understanding of
+ essentially simple, it is important to have a clear understanding of
the underlying techniques and assumptions.
</para>
<listitem><para>Continuous archiving</para></listitem>
</itemizedlist>
Each has its own strengths and weaknesses.
+ Each is discussed in turn below.
</para>
<sect1 id="backup-dump">
<para>
Dumps created by <application>pg_dump</> are internally consistent,
- that is, updates to the database while <application>pg_dump</> is
-
running will not be in the dump. <application>pg_dump</> does not
+ that is, the dump represents a snapshot of the database as of the time
+
<application>pg_dump</> begins running. <application>pg_dump</> does not
block other operations on the database while it is working.
(Exceptions are those operations that need to operate with an
- exclusive lock, such as <command>VACUUM FULL</command>.)
+ exclusive lock, such as most forms of <command>ALTER TABLE</command>.)
</para>
<important>
before executing <application>psql</> (e.g., with
<literal>createdb -T template0 <replaceable
class="parameter">dbname</></literal>). <application>psql</>
- supports similar options to <application>pg_dump</> for specifying
+ supports options similar to <application>pg_dump</>'s for specifying
the database server to connect to and the user name to use. See
the <xref linkend="app-psql"> reference page for more information.
</para>
<programlisting>
\set ON_ERROR_STOP
</programlisting>
- Either way, you will only have a partially restored
- dump. Alternatively, you can specify that the whole dump should be
+ Either way, you will have an only partially restored database.
+ Alternatively, you can specify that the whole dump should be
restored as a single transaction, so the restore is either fully
completed or fully rolled back. This mode can be specified by
passing the <option>-1</> or <option>--single-transaction</>
<para>
The ability of <application>pg_dump</> and <application>psql</> to
write to or read from pipes makes it possible to dump a database
- directly from one server to another; for example:
+ directly from one server to another, for example:
<programlisting>
pg_dump -h <replaceable>host1</> <replaceable>dbname</> | psql -h <replaceable>host2</> <replaceable>dbname</>
</programlisting>
<para>
The dumps produced by <application>pg_dump</> are relative to
<literal>template0</>. This means that any languages, procedures,
- etc. added to <literal>template1</> will also be dumped by
+ etc. added via <literal>template1</> will also be dumped by
<application>pg_dump</>. As a result, when restoring, if you are
using a customized <literal>template1</>, you must create the
empty database from <literal>template0</>, as in the example
psql -f <replaceable class="parameter">infile</replaceable> postgres
</synopsis>
(Actually, you can specify any existing database name to start from,
- but if you are reloading in an empty cluster then <literal>postgres</>
- should generally be used.) It is always necessary to have
+ but if you are reloading into an empty cluster then <literal>postgres</>
+ should usually be used.) It is always necessary to have
database superuser access when restoring a <application>pg_dumpall</>
dump, as that is required to restore the role and tablespace information.
If you use tablespaces, be careful that the tablespace paths in the
dump are appropriate for the new installation.
</para>
+
+ <para>
+ <application>pg_dumpall</> works by emitting commands to re-create
+ roles, tablespaces, and empty databases, then invoking
+ <application>pg_dump</> for each database. This means that while
+ each database will be internally consistent, the snapshots of
+ different databases might not be exactly in-sync.
+ </para>
</sect2>
<sect2 id="backup-dump-large">
be larger than the maximum size allowed by your system. Since
<application>pg_dump</> can write to the standard output, you can
use standard Unix tools to work around this possible problem.
+ There are several ways to do it:
</para>
<formalpara>
Reload with:
<programlisting>
-createdb <replaceable class="parameter">dbname</replaceable>
gunzip -c <replaceable class="parameter">filename</replaceable>.gz | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
Reload with:
<programlisting>
-createdb <replaceable class="parameter">dbname</replaceable>
cat <replaceable class="parameter">filename</replaceable>* | psql <replaceable class="parameter">dbname</replaceable>
</programlisting>
</para>
</formalpara>
<formalpara>
- <title>Use the custom dump format.</title>
+ <title>Use <application>pg_dump</>'s custom dump format.</title>
<para>
If <productname>PostgreSQL</productname> was built on a system with the
<application>zlib</> compression library installed, the custom dump
</programlisting>
A custom-format dump is not a script for <application>psql</>, but
- instead must be restored with <application>pg_restore</>.
+ instead must be restored with <application>pg_restore</>, for example:
+
+<programlisting>
+pg_restore -d <replaceable class="parameter">dbname</replaceable> <replaceable class="parameter">filename</replaceable>
+</programlisting>
+
See the <xref linkend="app-pgdump"> and <xref
linkend="app-pgrestore"> reference pages for details.
</para>
</formalpara>
+ <para>
+ For very large databases, you might need to combine <command>split</>
+ with one of the other two approaches.
+ </para>
+
</sect2>
</sect1>
The database server <emphasis>must</> be shut down in order to
get a usable backup. Half-way measures such as disallowing all
connections will <emphasis>not</emphasis> work
- (mainly because <command>tar</command> and similar tools do not take an
- atomic snapshot of the state of the file system at a point in
- time). Information about stopping the server can be found in
+ (in part because <command>tar</command> and similar tools do not take
+ an atomic snapshot of the state of the file system,
+ but also because of internal buffering within the server).
+ Information about stopping the server can be found in
<xref linkend="server-shutdown">. Needless to say that you
also need to shut down the server before restoring the data.
</para>
table and the associated <filename>pg_clog</filename> data
because that would render all other tables in the database
cluster useless. So file system backups only work for complete
- restoration of an entire database cluster.
+ backup and restoration of an entire database cluster.
</para>
</listitem>
</orderedlist>
However, a backup created in this way saves
the database files in a state where the database server was not
properly shut down; therefore, when you start the database server
- on the backed-up data, it will think the server had crashed
- and replay the WAL log. This is not a problem, just be aware of
+ on the backed-up data, it will think the previous server instance had
+ crashed and replay the WAL log. This is not a problem, just be aware of
it (and be sure to include the WAL files in your backup).
</para>
<para>
- If your database is spread across multiple file systems, there might not
- be any way to obtain exactly-simultaneous frozen snapshots of all
+ If your database is spread across multiple file systems, there might not
+ be any way to obtain exactly-simultaneous frozen snapshots of all
the volumes. For example, if your data files and WAL log are on different
disks, or if tablespaces are on different file systems, it might
- not be possible to use snapshot backup because the snapshots must be
- simultaneous.
+ not be possible to use snapshot backup because the snapshots
+ <emphasis>must</> be simultaneous.
Read your file system documentation very carefully before trusting
to the consistent-snapshot technique in such situations. The safest
approach is to shut down the database server for long enough to
</para>
<para>
- To recover successfully using continuous archiving (also called "online
- backup" by many database vendors), you need a continuous
+ To recover successfully using continuous archiving (also called
+ <quote>online backup</> by many database vendors), you need a continuous
sequence of archived WAL files that extends back at least as far as the
- start time of your backup. So to get started, you should setup and test
+ start time of your backup. So to get started, you should set up and test
your procedure for archiving WAL files <emphasis>before</> you take your
first base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.
In an abstract sense, a running <productname>PostgreSQL</> system
produces an indefinitely long sequence of WAL records. The system
physically divides this sequence into WAL <firstterm>segment
- files</>, which are normally 16MB apiece (although the size can be
- altered when building <productname>PostgreSQL</>). The segment
+ files</>, which are normally 16MB apiece (although the segment size
+
can be altered when building <productname>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
</para>
<para>
- When archiving WAL data, we want to capture the contents of each segment
+ 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 <quote>saving
you have a way of identifying the original name of each file), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with as much flexibility as possible,
- <productname>PostgreSQL</> tries not to make any assumptions about how
+ <productname>PostgreSQL</> tries not to make any assumptions about how
the archiving will be done. Instead, <productname>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
In <varname>archive_command</>,
any <literal>%p</> is replaced by the path name of the file to
archive, while any <literal>%f</> is replaced by the file name only.
- (The path name is relative to the working directory of the server,
+ (The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is something
archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'
</programlisting>
which will copy archivable WAL segments to the directory
- <filename>/mnt/server/archivedir</>. (This is an example, not a
+ <filename>/mnt/server/archivedir</>. (This is an example, not a
recommendation, and might not work on all platforms.)
</para>
<para>
While designing your archiving setup, consider what will happen if
- the archive command fails repeatedly because some aspect requires
+ 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
+ could occur if you write to tape without an autochanger; when the tape
fills, nothing further can be archived until the tape is swapped.
You should ensure that any error condition or request to a human operator
- is reported appropriately so that the situation can be
- resolved relatively quickly. The <filename>pg_xlog/</> directory will
+ is reported appropriately so that the situation can be
+ resolved reasonably quickly. The <filename>pg_xlog/</> directory will
continue to fill with WAL segment files until the situation is resolved.
+ (If the filesystem containing <filename>pg_xlog/</> fills up,
+ <productname>PostgreSQL</> will do a PANIC shutdown. No prior
+ transactions will be lost, but the database will be unavailable until
+ you free some space.)
</para>
<para>
<para>
The archive command is only invoked on completed WAL segments. Hence,
- if your server generates only little WAL traffic (or has slack periods
+ 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
of one of these statements, WAL would not contain enough information
for archive recovery. (Crash recovery is unaffected.) For
this reason, <varname>archive_mode</> can only be changed at server
- start. (<varname>archive_command</> can be changed with a
- configuration file reload, and setting it to <literal>''</> does
- prevent archiving.)
+ start. However, <varname>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 <varname>archive_command</> to the empty
+ string (<literal>''</>).
+ This will cause WAL files to accumulate in <filename>pg_xlog/</> until a
+ working <varname>archive_command</> is re-established.
</para>
</sect2>
</para>
<para>
- It does not matter which database within the cluster you connect to to
+ 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.
</para>
<para>
Once the WAL segment files used during the backup are archived, you are
done. The file identified by <function>pg_stop_backup</>'s result is
- the last segment that needs to be archived to complete the backup.
+ the last segment that needs to be archived to complete the backup.
Archival of these files will happen automatically, since you have
already configured <varname>archive_command</>. In many cases, this
happens fairly quickly, but you are advised to monitor your archival
system to ensure this has taken place so that you can be certain you
- have a complete backup.
+ have a complete backup.
</para>
</listitem>
</orderedlist>
GNU <application>tar</> return an error code indistinguishable from
a fatal error if a file was truncated while <application>tar</> was
copying it. Fortunately, GNU <application>tar</> versions 1.16 and
- later exits with <literal>1</> if a file was changed during the backup,
+ later exit with <literal>1</> if a file was changed during the backup,
and <literal>2</> for other errors.
</para>
nor between the end of the backup and <function>pg_stop_backup</>; a
few minutes' delay won't hurt anything. (However, if you normally run the
server with <varname>full_page_writes</> disabled, you might notice a drop
- in performance between <function>pg_start_backup</> and
+ in performance between <function>pg_start_backup</> and
<function>pg_stop_backup</>, since <varname>full_page_writes</> is
effectively forced on during backup mode.) You must ensure that these
steps are carried out in sequence without any possible
<literal>0000000100001234000055CD</> the backup history file will be
named something like
<literal>0000000100001234000055CD.007C9330.backup</>. (The second
- number in the file name stands for an exact position within the WAL
+ 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
The backup history file is just a small text file. It contains the
label string you gave to <function>pg_start_backup</>, as well as
the starting and ending times and WAL segments of the backup.
- If you used the label to identify where the associated dump file is kept,
+ 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.
</para>
<listitem>
<para>
If you have the space to do so,
- copy the whole cluster data directory and any tablespaces to a temporary
+ 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,
+ copies of your existing database. If you do not have enough space,
you need at the least to copy the contents of the <filename>pg_xlog</>
subdirectory of the cluster data directory, as it might contain logs which
were not archived before the system went down.
<para>
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,
+ <literal>root</>!) and with the right permissions. If you are using
+ tablespaces,
you should verify that the symbolic links in <filename>pg_tblspc/</>
were correctly restored.
</para>
Remove any files present in <filename>pg_xlog/</>; these came from the
backup dump and are therefore probably obsolete rather than current.
If you didn't archive <filename>pg_xlog/</> at all, then recreate it,
- and be sure to recreate the subdirectory
- <filename>pg_xlog/archive_status/</> as well.
+ being careful to ensure that you re-establish it as a symbolic link
+ if you had it set up that way before.
+ Be sure to recreate the subdirectory
+ <filename>pg_xlog/archive_status/</> as well.
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <filename>recovery.conf</> in the cluster
data directory (see <xref linkend="recovery-config-settings">). You might
- also want to temporarily modify <filename>pg_hba.conf</> to prevent
+ also want to temporarily modify <filename>pg_hba.conf</> to prevent
ordinary users from connecting until you are sure the recovery has worked.
</para>
</listitem>
</para>
<para>
- The key part of all this is to setup a recovery command file that
+ 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 <filename>recovery.conf.sample</> (normally
installed in the installation <filename>share/</> directory) as a
a shell command string. It can contain <literal>%f</>, which is
replaced by the name of the desired log file, and <literal>%p</>,
which is replaced by the path name to copy the log file to.
- (The path name is relative to the working directory of the server,
+ (The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to the current point in time (or as
close as we can get given the available WAL segments). So a normal
- recovery will end with a "file not found" message, the exact text
- of the error message depending upon your choice of
+ recovery will end with a <quote>file not found</> message, the exact text
+ of the error message depending upon your choice of
<varname>restore_command</>. You may also see an error message
at the start of recovery for a file named something like
<filename>00000001.history</>. This is also normal and does not
- indicate a problem in simple recovery situations. See
+ indicate a problem in simple recovery situations. See
<xref linkend="backup-timelines"> for discussion.
</para>
<para>
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 <filename>recovery.conf</>. You can specify
- the stop point, known as the <quote>recovery target</>, either by
- date/time or by completion of a specific transaction ID. As of this
- writing only the date/time option is very usable, since there are no tools
+ the junior DBA dropped your main transaction table), just specify the
+ required stopping point in <filename>recovery.conf</>. You can specify
+ the stop point, known as the <quote>recovery target</>, either by
+ date/time or by completion of a specific transaction ID. As of this
+ writing only the date/time option is very usable, since there are no tools
to help you identify with any accuracy which transaction ID to use.
</para>
<note>
<para>
- The stop point must be after the ending time of the base backup (the
- time of <function>pg_stop_backup</>). You cannot use a base backup
+ The stop point must be after the ending time of the base backup, i.e.,
+ the time of <function>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.)
<para>
If recovery finds a corruption in the WAL data then recovery will
complete at that point and the server will not start. In such a case the
- recovery process could be re-run from the beginning, specifying a
+ recovery process could be re-run from the beginning, specifying a
<quote>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
replaced by the name of the file to retrieve from the archive,
and any <literal>%p</> is replaced by the path name to copy
it to on the server.
- (The path name is relative to the working directory of the server,
+ (The path name is relative to the current working directory,
i.e., the cluster's data directory.)
Any <literal>%r</> is replaced by the name of the file containing the
last valid restart point. That is the earliest file that must be kept
to allow a restore to be restartable, so this information can be used
to truncate the archive to just the minimum required to support
restart of the current restore. <literal>%r</> would only be used in a
- warm-standby configuration (see <xref
- linkend="warm-standby-planning">).
+ warm-standby configuration (see <xref linkend="warm-standby">).
Write <literal>%%</> to embed an actual <literal>%</> character
in the command.
</para>
</varlistentry>
<varlistentry id="recovery-target-time" xreflabel="recovery_target_time">
- <term><varname>recovery_target_time</varname>
+ <term><varname>recovery_target_time</varname>
(<type>timestamp</type>)
</term>
<listitem>
At most one of <varname>recovery_target_time</> and
<xref linkend="recovery-target-xid"> can be specified.
The default is to recover to the end of the WAL log.
- The precise stopping point is also influenced by
+ The precise stopping point is also influenced by
<xref linkend="recovery-target-inclusive">.
</para>
</listitem>
<listitem>
<para>
This parameter specifies the transaction ID up to which recovery
- will proceed. Keep in mind
- that while transaction IDs are assigned sequentially at transaction
+ will proceed. Keep in mind
+ that while transaction IDs are assigned sequentially at transaction
start, transactions can complete in a different numeric order.
The transactions that will be recovered are those that committed
before (and optionally including) the specified one.
At most one of <varname>recovery_target_xid</> and
<xref linkend="recovery-target-time"> can be specified.
The default is to recover to the end of the WAL log.
- The precise stopping point is also influenced by
+ The precise stopping point is also influenced by
<xref linkend="recovery-target-inclusive">.
</para>
</listitem>
</varlistentry>
- <varlistentry id="recovery-target-inclusive"
+ <varlistentry id="recovery-target-inclusive"
xreflabel="recovery_target_inclusive">
- <term><varname>recovery_target_inclusive</varname>
+ <term><varname>recovery_target_inclusive</varname>
(<type>boolean</type>)
</term>
<listitem>
<para>
Specifies whether we stop just after the specified recovery target
- (<literal>true</literal>), or just before the recovery target
+ (<literal>true</literal>), or just before the recovery target
(<literal>false</literal>).
Applies to both <xref linkend="recovery-target-time">
and <xref linkend="recovery-target-xid">, whichever one is
</listitem>
</varlistentry>
- <varlistentry id="recovery-target-timeline"
+ <varlistentry id="recovery-target-timeline"
xreflabel="recovery_target_timeline">
- <term><varname>recovery_target_timeline</varname>
+ <term><varname>recovery_target_timeline</varname>
(<type>string</type>)
</term>
<listitem>
</listitem>
</varlistentry>
- <varlistentry id="log-restartpoints"
+ <varlistentry id="log-restartpoints"
xreflabel="log_restartpoints">
- <term><varname>log_restartpoints</varname>
+ <term><varname>log_restartpoints</varname>
(<type>boolean</type>)
</term>
<listitem>
<para>
- Specifies whether to log each restart point as it occurs. This
+ Specifies whether to log each restart point as it occurs. This
can be helpful to track the progress of a long recovery.
Default is <literal>false</>.
</para>
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.
+ perhaps 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 <emphasis>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 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
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
<title>Tips and Examples</title>
<para>
- Some examples of configuring Continuous Archiving are given here.
+ Some tips for configuring continuous archiving are given here.
</para>
<sect3 id="backup-standalone">
- <title>Recovery Settings</title>
+ <title>Standalone hot backups</title>
<para>
- It is possible to use the existing backup facilities to produce
- standalone hot backups. These are backups that cannot be used for
- point-in-time recovery, yet are much faster to backup and restore
- than <application>pg_dump</>.
+ It is possible to use <productname>PostgreSQL</>'s backup facilities to
+ produce standalone hot backups. These are backups that cannot be used
+ for point-in-time recovery, yet are typically much faster to backup and
+ restore than <application>pg_dump</> dumps. (They are also much larger
+ than <application>pg_dump</> dumps, so in some cases the speed advantage
+ could be negated.)
</para>
<para>
- To configure standalone backups you should use a switch file. If the
- file exists then archives are made, otherwise archiving is ignored.
+ To prepare for standalone hot backups, set <varname>archive_mode</> to
+ <literal>on</>, and set up an <varname>archive_command</> that performs
+ archiving only when a <quote>switch file</> exists. For example:
<programlisting>
archive_command = 'if [ -f /var/lib/pgsql/backup_in_progress ]; then cp -i %p /var/lib/pgsql/archive/%f </dev/null ; fi'
</programlisting>
- Backup can then be taken using a script like the following:
+ This command will perform archiving when
+ <filename>/var/lib/pgsql/backup_in_progress</> exists, and otherwise
+ silently return zero exit status (allowing <productname>PostgreSQL</>
+ to recycle the unwanted WAL file).
+ </para>
+
+ <para>
+ With this preparation, a backup can be taken using a script like the
+ following:
<programlisting>
touch /var/lib/pgsql/backup_in_progress
psql -c "select pg_start_backup('hot_backup');"
-tar -cvf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
+tar -cf /var/lib/pgsql/backup.tar /var/lib/pgsql/data/
psql -c "select pg_stop_backup();"
sleep 20
rm /var/lib/pgsql/backup_in_progress
-tar -rvf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
+tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
</programlisting>
The switch file <filename>/var/lib/pgsql/backup_in_progress</> is
- created first, allowing archiving to start prior to the backup.
+ created first, enabling archiving of completed WAL files to occur.
After the backup the switch file is removed. Archived WAL files are
then added to the backup so that both base backup and all required
WAL files are part of the same <application>tar</> file.
<title><varname>archive_command</varname> scripts</title>
<para>
- Many people choose to use scripts to define their
+ Many people choose to use scripts to define their
<varname>archive_command</varname>, so that their
- <filename>postgresql.conf</> looks very simple:
+ <filename>postgresql.conf</> entry looks very simple:
<programlisting>
archive_command = 'local_backup_script.sh'
</programlisting>
+ Using a separate script file is advisable any time you want to use
+ more than a single command in the archiving process.
This allows all complexity to be managed within the script, which
can be written in a popular scripting language such as
- <application>bash</> or <application>perl</>. Statements echoed to
- <literal>stderr</> will appear in the database server log, allowing
- complex configurations to be easily diagnosed if they fail.
+ <application>bash</> or <application>perl</>.
+ Any messages written to <literal>stderr</> from the script will appear
+ in the database server log, allowing complex configurations to be
+ diagnosed easily if they fail.
</para>
+
<para>
- Example of how scripts might be used include:
+ Examples of requirements that might be solved within a script include:
<itemizedlist>
<listitem>
<para>
- Copying data to a secure off-site data storage provider
+ Copying data to secure off-site data storage
</para>
</listitem>
<listitem>
<para>
- Batching WAL files so they are transferred every three hours, rather than
- one at a time as they fill
+ Batching WAL files so that they are transferred every three hours,
+ rather than one at a time
</para>
</listitem>
<listitem>
</listitem>
<listitem>
<para>
- Interfacing with monitoring software to report errors directly
+ Interfacing with monitoring software to report errors
</para>
</listitem>
</itemizedlist>
</para>
<para>
- Directly moving WAL or "log" records from one database server to another
+ Directly moving WAL records from one database server to another
is typically described as log shipping. <productname>PostgreSQL</>
implements file-based log shipping, which means that WAL records are
transferred one file (WAL segment) at a time. WAL files (16MB) can be
capability as a warm standby configuration that offers high
availability. Restoring a server from an archived base backup and
rollforward will take considerably longer, so that technique only
- really offers a solution for disaster recovery, not high availability.
+ offers a solution for disaster recovery, not high availability.
</para>
<sect2 id="warm-standby-planning">
</para>
<para>
- In general, log shipping between servers running different major release
+ In general, log shipping between servers running different major
+ <productname>PostgreSQL</> release
levels will not be possible. It is the policy of the PostgreSQL Global
Development Group not to make changes to disk formats during minor release
- upgrades, so it is likely that running different minor release levels
+ upgrades, so it is likely that running different minor release levels
on primary and standby servers will work successfully. However, no
formal support for that is offered and you are advised to keep primary
and standby servers at the same release level as much as possible.
<para>
A working example of a waiting <varname>restore_command</> is provided
- as a contrib module, named <application>pg_standby</>. This can be
- extended as needed to support specific configurations or environments.
+ as a <filename>contrib</> module named <application>pg_standby</>. This
+ example can be extended as needed to support specific configurations or
+ environments.
</para>
<para>
time as it is being read by the standby database server.
Thus, running a standby server for high availability can be performed at
the same time as files are stored for longer term disaster recovery
- purposes.
+ purposes.
</para>
<para>
<para>
If the standby server fails then no failover need take place. If the
standby server can be restarted, even some time later, then the recovery
- process can also be immediately restarted, taking advantage of
+ process can also be immediately restarted, taking advantage of
restartable recovery. If the standby server cannot be restarted, then a
- full new standby server should be created.
+ full new standby server instance should be created.
</para>
<para>
a mechanism for informing it that it is no longer the primary. This is
sometimes known as STONITH (Shoot the Other Node In The Head), which is
necessary to avoid situations where both systems think they are the
- primary, which can lead to confusion and ultimately data loss.
+ primary, which will lead to confusion and ultimately data loss.
</para>
<para>
Many failover systems use just two systems, the primary and the standby,
connected by some kind of heartbeat mechanism to continually verify the
connectivity between the two and the viability of the primary. It is
- also possible to use a third system (called a witness server) to avoid
- some problems of inappropriate failover, but the additional complexity
- might not be worthwhile unless it is set-up with sufficient care and
+ also possible to use a third system (called a witness server) to prevent
+ some cases of inappropriate failover, but the additional complexity
+ might not be worthwhile unless it is set up with sufficient care and
rigorous testing.
</para>
<para>
Once failover to the standby occurs, we have only a
single server in operation. This is known as a degenerate state.
- The former standby is now the primary, but the former primary is down
+ The former standby is now the primary, but the former primary is down
and might stay down. To return to normal operation we must
- fully recreate a standby server,
- either on the former primary system when it comes up, or on a third,
- possibly new, system. Once complete the primary and standby can be
- considered to have switched roles. Some people choose to use a third
+ fully recreate a standby server,
+ either on the former primary system when it comes up, or on a third,
+ possibly new, system. Once complete the primary and standby can be
+ considered to have switched roles. Some people choose to use a third
server to provide backup to the new primary until the new standby
server is recreated,
- though clearly this complicates the system configuration and
+ though clearly this complicates the system configuration and
operational processes.
</para>
<para>
So, switching from primary to standby server can be fast but requires
some time to re-prepare the failover cluster. Regular switching from
- primary to standby is encouraged, since it allows regular downtime on
- each system for maintenance. This also acts as a test of the
- failover mechanism to ensure that it will really work when you need it.
+ primary to standby is useful, since it allows regular downtime on
+ each system for maintenance. This also serves as a test of the
+ failover mechanism to ensure that it will really work when you need it.
Written administration procedures are advised.
</para>
</sect2>
over to the standby server(s). With this approach, the window for data
loss is the polling cycle time of the copying program, which can be very
small, but there is no wasted bandwidth from forcing partially-used
- segment files to be archived. Note that the standby servers'
+ segment files to be archived. Note that the standby servers'
<varname>restore_command</> scripts still deal in whole WAL files,
so the incrementally copied data is not ordinarily made available to
the standby servers. It is of use only when the primary dies —
In a warm standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups
can be made by backing
- up a standby server's files. This concept is generally known as
- incrementally updated backups, log change accumulation or more simply,
+ up a standby server's files. This concept is generally known as
+ incrementally updated backups, log change accumulation, or more simply,
change accumulation.
</para>
far back you need to keep WAL segment files to have a recoverable
backup. You can do this by running <application>pg_controldata</>
on the standby server to inspect the control file and determine the
- current checkpoint WAL location, or by using the
+ current checkpoint WAL location, or by using the
<varname>log_restartpoints</> option to print values to the server log.
</para>
</sect2>
the number after the first dot changes). This does not apply to
different minor releases under the same major release (where the
number after the second dot changes); these always have compatible
- storage formats. For example, releases 7.2.1, 7.3.2, and 7.4 are
- not compatible, whereas 7.2.1 and 7.2.2 are. When you update
+ storage formats. For example, releases 8.1.1, 8.2.3, and 8.3 are
+ not compatible, whereas 8.2.3 and 8.2.4 are. When you update
between compatible versions, you can simply replace the executables
and reuse the data directory on disk. Otherwise you need to back
up your data and restore it on the new server. This has to be done
to transfer your data. Or use an intermediate file if you want.
Then you can shut down the old server and start the new server at
the port the old one was running at. You should make sure that the
- old database is not updated after you run <application>pg_dumpall</>,
- otherwise you will obviously lose that data. See <xref
+ old database is not updated after you begin to run
+ <application>pg_dumpall</>, otherwise you will lose that data. See <xref
linkend="client-authentication"> for information on how to prohibit
access.
</para>
<para>
- It is also possible to use replication like <productname>Slony</> to
- create a slave server with the updated version of
+ It is also possible to use replication methods, such as
+ <productname>Slony</>, to create a slave server with the updated version of
<productname>PostgreSQL</>. The slave can be on the same computer or
a different computer. Once it has synced up with the master server
(running the older version of <productname>PostgreSQL</>), you can
</para>
<para>
- If you cannot or do not want to run two servers in parallel you can
+ If you cannot or do not want to run two servers in parallel, you can
do the backup step before installing the new version, bring down
the server, move the old version out of the way, install the new
- version, start the new server, restore the data. For example:
+ version, start the new server, and restore the data. For example:
<programlisting>
pg_dumpall > backup
When you <quote>move the old installation out of the way</quote>
it might no longer be perfectly usable. Some of the executable programs
contain absolute paths to various installed programs and data files.
- This is usually not a big problem but if you plan on using two
+ This is usually not a big problem, but if you plan on using two
installations in parallel for a while you should assign them
different installation directories at build time. (This problem
- is rectified in <productname>PostgreSQL</> 8.0 and later, but you
- need to be wary of moving older installations.)
+ is rectified in <productname>PostgreSQL</> 8.0 and later, so long
+ as you move all subdirectories containing installed files together;
+ for example if <filename>/usr/local/postgres/bin/</> goes to
+ <filename>/usr/local/postgres.old/bin/</>, then
+ <filename>/usr/local/postgres/share/</> must go to
+ <filename>/usr/local/postgres.old/share/</>. In pre-8.0 releases
+ moving an installation like this will not work.)
</para>
</note>
</sect1>
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