<p>This user guide is intended to be followed sequentially from beginning to end &mdash; each section depends on the last. For example the <linksection="/backup">Backup</link> section relies on setup that is performed in the <linksection="/quickstart">Quick Start</link> section. Once you have <backrest/> up and running then skipping around is possible but it is recommended to follow the user guide in order the first time through.</p>
<p>Although the examples are targeted at Ubuntu and <postgres/> 9.4 they will also work fine on Debian and it should be fairly easy to apply this guide to any Unix distribution and <postgres/> version. The only OS-specific commands are those to create, start, stop, and drop <postgres/> clusters. The <backrest/> commands will be the same on any Unix system though the locations to install Perl libraries and executables may vary. Configuring archiving is different on <postgres/> versions &lt;= 8.4 and configuration information can be found in the <postgres/> documentation.</p>
<p>A somewhat novel approach is taken to documentation in this user guide. Each command is run on a virtual machine when the documentation is built from the XML source. This means you can have a high confidence that the commands work correctly in the order presented. Output is captured and displayed below the command when appropriate. If the output is not included it is because it was deemed not relevant or was considered a distraction from the narrative.</p>
<p>All commands are intended to be run as an unprivileged user that has sudo privileges for both the <user>root</user> and <user>postgres</user> users. It's also possible to run the commands directly as their respective users without modification and in that case the <cmd>sudo</cmd> commands can be stripped off.</p>
<p>A backup is a consistent copy of a database cluster that can be restored to recover from a hardware failure, to perform Point-In-Time Recovery, or to bring up a new standby.</p>
<p><b>Full Backup</b>: <backrest/> copies the entire contents of the database cluster to the backup server. The first backup of the database cluster is always a Full Backup. <backrest/> is always able to restore a full backup directly. The full backup does not depend on any files outside of the full backup for consistency.</p>
<p><b>Differential Backup</b>: <backrest/> copies only those database cluster files that have changed since the last full backup. <backrest/> restores a differential backup by copying all of the files in the chosen differential backup and the appropriate unchanged files from the previous full backup. The advantage of a differential backup is that it requires less disk space than a full backup, however, the differential backup and the full backup must both be valid to restore the differential backup.</p>
<p><b>Incremental Backup</b>: <backrest/> copies only those database cluster files that have changed since the last backup (which can be another incremental backup, a differential backup, or a full backup). As an incremental backup only includes those files changed since the prior backup, they are generally much smaller than full or differential backups. As with the differential backup, the incremental backup depends on other backups to be valid to restore the incremental backup. Since the incremental backup includes only those files since the last backup, all prior incremental backups back to the prior differential, the prior differential backup, and the prior full backup must all be valid to perform a restore of the incremental backup. If no differential backup exists then all prior incremental backups back to the prior full backup, which must exist, and the full backup itself must be valid to restore the incremental backup.</p>
<p>A restore is the act of copying a backup to a system where it will be started as a live database cluster. A restore requires the backup files and one or more WAL segments in order to work correctly.</p>
<p>WAL is the mechanism that <postgres/> uses to ensure that no committed changes are lost. Transactions are written sequentially to the WAL and a transaction is considered to be committed when those writes are flushed to disk. Afterwards, a background process writes the changes into the main database cluster files (also known as the heap). In the event of a crash, the WAL is replayed to make the database consistent.</p>
<p>WAL is conceptually infinite but in practice is broken up into individual 16MB files called segments. WAL segments follow the naming convention <id>0000000100000A1E000000FE</id> where the first 8 hexadecimal digits represent the timeline and the next 16 digits are the logical sequence number (LSN).</p>
<pkeyword="default"><backrest/> is written in Perl which is included with Ubuntu by default. A few additional modules are required which are all available as packages.</p>
<pkeyword="co6"><backrest/> is written in Perl, which is not included with RHEL/CentOS by default, however all required modules are all available as standard packages.</p>
<p><backrest/> should now be properly installed but it is best to check. If any dependencies were missed then you will get an error when running <backrest/> from the command line.</p>
<p>The Quick Start section will cover basic configuration of <backrest/> and <postgres/> and introduce the <cmd>backup</cmd>, <cmd>restore</cmd>, and <cmd>info</cmd> commands.</p>
<p>Creating the demo cluster is optional but commands in the user guide may not work in other environments without significant modification. The cluster won't be started immediately because there is still some configuration to do.</p>
<p>By default <postgres/> will only accept local connections. The examples in this guide will require connections from other servers so <pg-option>listen_addresses</pg-option> is configured to listen on all interfaces. This may not be appropriate for secure installations.</p>
<p>For demonstration purposes the <pg-option>log_line_prefix</pg-option> setting will be minimally configured. This keeps the log output as brief as possible to better illustrate important information.</p>
<pkeyword="co6">By default CentOS/RHEL includes the day of the week in the log filename. This makes automating the user guide a bit more complicated so the <pg-option>log_filename</pg-option> is set to a constant.</p>
<p>Demo describes the purpose of this cluster accurately so that will also make a good stanza name.</p>
<p><backrest/> needs to know where the base data directory for the <postgres/> cluster is located. The path can be requested from <postgres/> directly but in a recovery scenario the <postgres/> process will not be available. During backups the value supplied to <backrest/> will be compared against the path that <postgres/> is running on and they must be equal or the backup will return an error. Make sure that <br-option>db-path</br-option> is exactly equal to <pg-option>data_directory</pg-option> in <file>postgresql.conf</file>.</p>
<p>By default Ubuntu stores clusters in <path>/var/lib/postgresql/[version]/[cluster]</path> so it is easy to determine the correct path for the data directory.</p>
<p><backrest/> configuration files follow the Windows INI convention. Sections are denoted by text in brackets and key/value pairs are contained in each section. Lines beginning with <id>#</id> are ignored and can be used as comments.</p>
<!-- <p>For the purposes of demonstration set console logging to <br-setting>info</br-setting> (but leave the archive command at <br-setting>warn</br-setting>). <backrest/> will output useful information what will be used to illustrate examples.</p>
<p>For this demonstration the repository will be stored on the same host as the <postgres/> server. This is the simplest configuration and is useful in cases where traditional backup software is employed to backup the database host.</p>
<p>The <pg-option>wal_level</pg-option> setting must be set to <pg-setting>archive</pg-setting> at a minimum but <pg-setting>hot_standby</pg-setting> and <pg-setting>logical</pg-setting> also work fine for backups. Setting <pg-option>wal_level</pg-option> to <pg-setting>hot_standy</pg-setting> and increasing <pg-option>max_wal_senders</pg-option> is a good idea even if you do not currently run a hot standby as this will allow them to be added later without restarting the master cluster.</p>
<p>By default <backrest/> will attempt to perform an incremental backup. However, an incremental backup must be based on a full backup and since no full backup existed <backrest/> ran a full backup instead.</p>
<p>This time there was no warning because a full backup already existed. While incremental backups can be based on a full <i>or</i> differential backup, differential backups must be based on a full backup. A full backup can be performed by running the <cmd>backup</cmd> command with <br-setting>{[dash]}-type=full</br-setting>.</p>
<p>The oldest and newest backups are shown in the info output. The oldest backup will <i>always</i> be a full backup (indicated by an <id>F</id> at the end of the label) but the newest backup can be full, differential (ends with <id>D</id>), or incremental (ends with <id>I</id>).</p>
<p>Backups can protect you from a number of disaster scenarios, the most common of which are hardware failure and data corruption. The easiest way to simulate data corruption is to remove an important <postgres/> cluster file.</p>
<p>To restore a backup of the <postgres/> cluster run <backrest/> with the <cmd>restore</cmd> command. The cluster needs to be stopped (in this case it is already stopped) and all files must be removed from the <postgres/> data directory.</p>
<p>By default <backrest/> will wait for the next regularly scheduled checkpoint before starting a backup. Depending on the <pg-option>checkpoint_timeout</pg-option> and <pg-option>checkpoint_segments</pg-option> settings in <postgres/> it may be quite some time before a checkpoint completes and the backup can begin.</p>
<exe-highlight>backup begins after the next regular checkpoint completes</exe-highlight>
</execute>
</execute-list>
<p>By setting <br-setting>start-fast</br-setting> on the command-line or in <file>{[backrest-config-demo]}</file> an immediate checkpoint is requested and the backup will start more quickly. This is convenient for testing and for ad-hoc backups. For instance, if a backup is being taken at the beginning of a release window it makes no sense to wait for a checkpoint. Since regularly scheduled backups generally only happen once per day it is unlikely that enabling the <br-option>start-fast</br-option> in <file>{[backrest-config-demo]}</file> will negatively affect performance, however for high-volume transactional systems you may want to pass <br-setting>{[dash]}-start-fast</br-setting> on the command-line instead.</p>
<p>Sometimes <backrest/> will exit unexpectedly and the backup in progress on the <postgres/> cluster will not be properly stopped. <backrest/> exits as quickly as possible when an error occurs so that the cause can be reported accurately and is not masked by another problem that might happen during a more extensive cleanup.</p>
<p>Even when the permissions are fixed <backrest/> will still be unable to perform a backup because the <postgres/> cluster is stuck in backup mode.</p>
<p>Enabling the <br-option>stop-auto</br-option> option allows <backrest/> to stop the current backup if it detects that no other <backrest/> backup process is running.</p>
<exe-highlight>cluster is already in backup mode|backup begins after the requested immediate checkpoint completes</exe-highlight>
</execute>
</execute-list>
<p>Although useful this feature may not be appropriate when another third-party backup solution is being used to take online backups as <backrest/> will not recognize that the other software is running and may terminate a backup started by that software. However, it would be unusual to run more than one third-party backup solution at the same time so this is not likely to be a problem.</p>
<p>Note that <id>pg_dump</id> and <id>pg_base_backup</id> do not take online backups so are not affected. It is safe to run them in conjunction with <backrest/>.</p>
<p>Generally it is best to retain as many backups as possible to provide a greater window for <linksection="/pitr">Point-in-Time Recovery</link>, but practical concerns such as disk space must also be considered. Retention options remove older backups once they are no longer needed.</p>
<p>Set <br-option>full-retention</br-option> to the number of full backups required. New backups must be completed before expiration will occur &mdash; that means if <br-setting>retention-full=2</br-setting> then there will be three full backups stored before the oldest one is expired.</p>
<p>Backup <br-setting>retention-full=2</br-setting> but currently there is only one full backup so the next full backup to run will not expire any full backups.</p>
<p>Archive <i>is</i> expired because WAL segments were generated before the oldest backup. These are not useful for recovery &mdash; only WAL segments generated after a backup can be used to recover that backup.</p>
<p>The <id>{[backup-full-first]}</id> full backup is expired and archive retention is based on the <id>{[backup-full-second]}</id> which is now the oldest full backup.</p>
</section>
<!-- SECTION => RETENTION - DIFF -->
<sectionid="diff">
<title>Differential Backup Retention</title>
<p>Set <br-option>retention-diff</br-option> to the number of differential backups required. Differentials only rely on the prior full backup so it is possible to create a <quote>rolling</quote> set of differentials for the last day or more. This allows quick restores to recent points-in-time but reduces overall space consumption.</p>
<p>Backup <br-setting>retention-diff=1</br-setting> so two differentials will need to be performed before one is expired. An incremental backup is added to demonstrate incremental expiration. Incremental backups cannot be expired independently &mdash; they are always expired with their related full or differential backup.</p>
<p><linksection="/quickstart/perform-restore">Restore a Backup</link> in <linksection="/quickstart">Quick Start</link> required the database cluster directory to be cleaned before the <cmd>restore</cmd> could be performed. The <br-option>delta</br-option> option allows <backrest/> to automatically determine which files in the database cluster directory can be preserved and which ones need to be restored from the backup &mdash; it also <i>removes</i> files not present in the backup manifest so it will dispose of divergent changes. This is accomplished by calculating a <linkurl="https://en.wikipedia.org/wiki/SHA-1">SHA-1</link> cryptographic hash for each file in the database cluster directory. If the <id>SHA-1</id> hash does not match the hash stored in the backup then that file will be restored. This operation is very efficient when combined with the <br-option>thread-max</br-option> option. Since the <postgres/> process is shut down during the restore, a larger number of threads can be used than might be desirable during a backup when the <postgres/> process is running.</p>
<p><linksection="/quickstart/perform-restore">Restore a Backup</link> in <linksection="/quickstart">Quick Start</link> performed default recovery, which is to play all the way to the end of the WAL stream. In the case of a hardware failure this is usually the best choice but for data corruption scenarios (whether machine or human in origin) Point-in-Time Recovery (PITR) is often more appropriate.</p>
<p>Point-in-Time Recovery (PITR) allows the WAL to be played from the last backup to a specified time, transaction id, or recovery point. For common recovery scenarios time-based recovery is arguably the most useful. A typical recovery scenario is to restore a table that was accidentally dropped or data that was accidentally deleted. Recovering a dropped table is more dramatic so that's the example given here but deleted data would be recovered in exactly the same way.</p>
<p>It is important to represent the time as reckoned by <postgres/> and to include timezone offsets. This reduces the possibility of unintended timezone conversions and an unexpected recovery result.</p>
<p>Now that the time has been recorded the table is dropped. In practice finding the exact time that the table was dropped is a lot harder than in this example. It may not be possible to find the exact time, but some forensic work should be able to get you close.</p>
<execute-listhost="{[host-db-master]}">
<title>Drop the important table</title>
<executeoutput="y"err-expect="1">
<exe-cmd>psql -c "begin;
drop table important_table;
commit;
select * from important_table;"</exe-cmd>
<exe-highlight>does not exist</exe-highlight>
</execute>
</execute-list>
<p>Now the restore can be performed with time-based recovery to bring back the missing table.</p>
<execute-listhost="{[host-db-master]}">
<title>Stop <postgres/>, restore the {[postgres-cluster-demo]} cluster to <id>{[time-recovery-timestamp]}</id>, and display <file>recovery.conf</file></title>
<p>The <file>recovery.conf</file> file has been automatically generated by <backrest/> so <postgres/> can be started immediately. Once <postgres/> has finished recovery the table will exist again and can be queried.</p>
<execute-listhost="{[host-db-master]}">
<title>Start <postgres/> and check that the important table exists</title>
<p>The <postgres/> log also contains valuable information. It will indicate the time and transaction where the recovery stopped and also give the time of the last transaction to be applied.</p>
<p>This example was rigged to give the correct result. If a backup after the required time is chosen then <postgres/> will not be able to recover the lost table. <postgres/> can only play forward, not backward. To demonstrate this the important table must be dropped (again).</p>
<execute-listhost="{[host-db-master]}">
<title>Drop the important table (again)</title>
<executeoutput="y"err-expect="1">
<exe-cmd>psql -c "begin;
drop table important_table;
commit;
select * from important_table;"</exe-cmd>
<exe-highlight>does not exist</exe-highlight>
</execute>
</execute-list>
<p>Now take a new backup and attempt recovery from the new backup.</p>
<execute-listhost="{[host-db-master]}">
<title>Perform a backup then attempt recovery from that backup</title>
<exe-cmd>psql -c "select * from important_table"</exe-cmd>
<exe-highlight>does not exist</exe-highlight>
</execute>
</execute-list>
<p>Looking at the log output it's not obvious that recovery failed to restore the table. The key is to look for the presence of the <quote>recovery stopping before...</quote> and <quote>last completed transaction...</quote> log messages. If they are not present then the recovery to the specified point-in-time was not successful.</p>
<execute-listhost="{[host-db-master]}">
<title>Examine the <postgres/> log output to discover the recovery was not successful</title>
<p>Using an earlier backup will allow <postgres/> to play forward to the correct time again. The default behavior for restore is to use the most recent backup but an earlier backup can be specified with the <br-option>{[dash]}-set</br-option> option.</p>
<execute-listhost="{[host-db-master]}">
<title>Stop <postgres/>, restore from the previous backup, and start <postgres/></title>
<p>Now the the log output will contain the expected <quote>recovery stopping before...</quote> and <quote>last completed transaction...</quote> messages showing that the recovery was successful.</p>
<execute-listhost="{[host-db-master]}">
<title>Examine the <postgres/> log output for log messages indicating success</title>
<p>The configuration described in <linksection="/quickstart">Quickstart</link> is suitable for simple installations but for enterprise configurations it is more typical to have a dedicated <host>backup</host> host. This separates the backups and WAL archive from the database server so <host>database</host> host failures have less impact. It is still a good idea to employ traditional backup software to backup the <host>backup</host> host.</p>
<p>For this example a new host named <host>backup</host> has been created to store the cluster backups. Follow the instructions in <linksection="/installation">Installation</link> to install <backrest/> and <linksection="/quickstart/create-repository">Create the Repository</link> to create the <backrest/> repository. The <host>backup</host> host must also be configured with the database host/user and database path.</p>
<p>The repository directory will also be removed from the database host. It will not be used anymore so leaving it around may be confusing later on.</p>
<execute-listhost="{[host-db-master]}">
<title>Remove repository now that it will be located on the database server</title>
<p>Commands are run the same as on a single host configuration except that the <cmd>backup</cmd> and <cmd>expire</cmd> command are run from the <host>backup</host> host and all other commands are run from the <host>database</host> host.</p>
<p>Since a new repository was created on the <host>backup</host> host the warning about the incremental backup changing to a full backup was emitted.</p>
<p>Sometimes it is useful to prevent <backrest/> from running on a system. For example, when failing over from a master to a standby it's best to prevent <backrest/> from running on the old master in case <postgres/> gets restarted or can't be completely killed. This will also prevent <backrest/> from running on <id>cron</id>.</p>
<exe-highlight>stop file exists for all stanzas</exe-highlight>
</execute>
</execute-list>
<p>Specify the <br-option>--force</br-option> option to terminate any <backrest/> process that are currently running. If <backrest/> is already stopped then stopping again will generate a warning.</p>
<execute-listhost="{[host-db-master]}">
<title>Stop the <backrest/> services again</title>
<executeoutput="y"filter="n">
<exe-cmd>{[project-exe]} stop</exe-cmd>
</execute>
</execute-list>
<p>Start <backrest/> processes again with the <cmd>start</cmd> command.</p>
<execute-listhost="{[host-db-master]}">
<title>Start the <backrest/> services</title>
<execute>
<exe-cmd>{[project-exe]} start</exe-cmd>
</execute>
</execute-list>
<p>It is also possible to stop <backrest/> for a single stanza.</p>
<execute-listhost="{[host-db-master]}">
<title>Stop <backrest/> services for the <id>demo</id> stanza</title>
<p>Replication allows multiple copies of a <postgres/> cluster (called standbys) to be created from a single master. The standbys are useful for balancing reads and to provide redundancy in case the master host fails.</p>
<sectionid="hot-standby">
<title>Hot Standby</title>
<p>A hot standby performs replication using the WAL archive and allows read-only queries.</p>
<p>A new host named <host>db-standby</host> will be created to run the standby. Follow the instructions in <linksection="/installation">Installation</link> to install <backrest/>, <linksection="/quickstart/setup-demo-cluster">Setup Demo Cluster</link> to setup the demo cluster, and <linksection="/quickstart/create-repository">Create the Repository</link> to create the <backrest/> repository on the <host>db-standby</host> host.</p>
<p><backrest/> configuration is very similar to <host>db-master</host> except that the <pg-option>standby_mode</pg-option> setting will be enabled to keep the cluster in recovery mode when the end of the WAL stream has been reached.</p>
<p>Note that the <pg-setting>standby_mode</pg-setting> setting has been written into the <file>recovery.conf</file> file. Configuring recovery settings in <backrest/> means that the <file>recovery.conf</file> file does not need to be stored elsewhere since it will be properly recreated with each restore. The <br-setting>--type=preserve</br-setting> option can be used with the <cmd>restore</cmd> to leave the existing <file>recovery.conf</file> file in place if that behavior is preferred.</p>
<p>The <pg-setting>hot_standby</pg-setting> setting must be enabled before starting <postgres/> to allow read-only connections on <host>db-standby</host>. Otherwise, connection attempts will be refused.</p>
<p>The <postgres/> log gives valuable information about the recovery. Note especially that the cluster has entered standby mode and is ready to accept read-only connections.</p>
<execute-listhost="{[host-db-standby]}">
<title>Examine the <postgres/> log output for log messages indicating success</title>
<executeoutput="y">
<exe-cmd>cat {[postgres-log-demo]}</exe-cmd>
<exe-highlight>entering standby mode|database system is ready to accept read only connections</exe-highlight>
</execute>
</execute-list>
<p>An easy way to test that replication is properly configured is to create a table on <host>db-master</host>.</p>
<execute-listhost="{[host-db-master]}">
<title>Create a new table on the master</title>
<executeoutput="y">
<exe-cmd>
psql -c "
begin;
create table replicated_table (message text);
insert into replicated_table values ('{[test-table-data]}');
<p>So, what went wrong? Since <postgres/> is pulling WAL segments from the archive to perform replication, changes won't be seen on the standby until the WAL segment that contains those changes is pushed from <host>db-master</host>.</p>
<p>This can be done manually by calling <code>pg_switch_xlog()</code> which pushes the current WAL segment to the archive (a new WAL segment is created to contain further changes).</p>
<p>Instead of relying solely on the WAL archive, streaming replication makes a direct connection to the master and applies changes as soon as they are made on the master. This results in much less lag between the master and standby.</p>
<p>Streaming replication requires a user with the replication privilege.</p>
<execute-listhost="{[host-db-master]}">
<title>Create replication user</title>
<executeoutput="y"filter="n">
<exe-cmd>
psql -c "
create user replicator password 'jw8s0F4' replication";
<p>The <file>pg_hba.conf</file> file must be updated to allow the standby to connection as the replication user. Be sure to replace the IP address below with the actual IP address of your <host>db-master</host>. A reload will be required after modifying the <file>pg_hba.conf</file> file.</p>
<execute-listhost="{[host-db-master]}">
<title>Create <file>pg_hba.conf</file> entry for replication user</title>
<!-- <p>The <pg-option>max_wal_senders</pg-option> setting must be increased (the default is 0) to allow standbys to connect to the master. It will be set to 3 to allow more standbys to be created later. <postgres/> must restarted for this setting to take effect.</p>
<p>It is possible to configure a password in the <pg-option>primary_conninfo</pg-option> setting but using a <file>.pgpass</file> file is more flexible and secure.</p>
<execute-listhost="{[host-db-standby]}">
<title>Configure the replication password in the <file>.pgpass</file> file.</title>
<pkeyword="co6">By default CentOS/RHEL stores the <file>postgresql.conf</file> file in the <postgres/> data directory. That means the change made to <file>postgresql.conf</file> was overwritten by the last restore and the <pg-option>hot_standby</pg-option> setting must be enabled again. Other solutions to this problem are to store the <file>postgresql.conf</file> file elsewhere or to enable the <pg-option>hot_standby</pg-option> setting on the <host>db-master</host> host where it will be ignored.</p>
<exe-highlight>started streaming WAL from primary</exe-highlight>
</execute>
</execute-list>
<p>Now when a table is created on <host>db-master</host> it will appear on <host>db-standby</host> quickly and without the need to call <code>pg_switch_xlog()</code>.</p>
<execute-listhost="{[host-db-master]}">
<title>Create a new table on the master</title>
<executeoutput="y">
<exe-cmd>
psql -c "
begin;
create table stream_table (message text);
insert into stream_table values ('{[test-table-data]}');
