Backup and-recovery2

PostgreSQL Backup and
Recovery

Except where otherwise noted, this work is licensed under
http://creativecommons.org/licenses/by/3.0/

PG West, 2009 www.consistentstate.com
by Kevin Kempter kevink@consistentstate.com

Session Topics

● Overview
● Backup Options
● Restore Options
● Point in Time Recovery (PITR)
● Warm Standby


Backup Options

● pg_dump
● pg_dumpall


pg_dump

● A PostgreSQL database backup/dump utility
● Creates consistent backups (even if the db is in use)
● Non-blocking
● Multiple Output File Formats
● Highly Flexible Options


pg_dump
● Syntax:
pg_dump [options] dbname

● Connection Options:
pg_dump -h <host> -p <port> -U <user> -W
pg_dump --host=<host> --port=<port> --username=<user> --password

● Environment Variables:
PGDATABASE PGHOST PGPORT PGUSER


pg_dump

● Common Options:
● -a (--data-only) ● -s (--schema-only)
● -c (--clean) ● -t (--table=<tablename>)
● -C (--create) ● -T (--exclude-table=<tablename>)
● -d (--inserts) ● -v (--verbose)
● -N (--exclude-schema=<schema>) ● -F <format> (--format=<format>)
● -n (--schema=<schema>)


pg_dump

● File Format Options:
● Plain
● Creates a plain text SQL file (default)
-F p (--format=plain)
● Custom
● Create a custom compressed format (compatible with pg_restore)
-F c (--format=custom)
● Tar
● Creates a tar format file (also compatible with pg_restore)
-F t (--format=tar)


pg_dump Examples
● $ pg_dump -C --inserts prod1_db > prod1_db.sql
Creates a dump of insert statements including a create database statement

● $ pg_dump --data-only --table=customer -F c prod1_db > prod1_db.fc.dmp
Dump the customer table in a custom format from the prod1_db database

● $ pg_dump -S prod1_db > prod1_db.ddl_only.sql
Creates a DDL only dump of the prod1_db database

● $ pg_dump --schema=gold -F t prod1_db > prod1_db.gold_schema.dmp
Creates a dump of the gold schema in the prod1_db database in a tar format


pg_dumpall

● A PostgreSQL cluster (instance) backup/dump utility
● Creates consistent backups (even if the db is in use)
● Non-blocking
● Multiple Output File Formats
● Highly Flexible Options


pg_dumpall
● Syntax:
pg_dumpall [options]




pg_dumpall

● Common Options:

● -a (--data-only)
● -r (--roles-only)

● -c (--clean)
● -s (--schema-only)

● -D (--column-inserts) (--attribute-inserts)
● -S (--superuser=<username>)

● -d (--inserts)
● -v (--verbose)

● -g (--globals-only)
● -t (--tablespaces-only)

● -o (--oids)
● --disable-triggers


pg_dumpall Examples
● $ pg_dumpall -g > prod1_db.global_structures.sql
Creates a cluster dump containing only the cluster global structures

● $ pg_dumpall --tablespaces-only > prod1_db.tablespaces.sql
Dump the cluster tablespaces

● $ pg_dumpall -r > prod1_db.roles_only.sql
Creates a dump of only the cluster roles

● $ pg_dumpall -o -S gold_user > prod1_db.oids.dmp
Creates a dump of the cluster including oid's as the superuser 'gold_user'


Restore Options

● pg_restore
● psql


pg_restore
● Syntax:
pg_restore [options] [filename]




pg_restore
● Common Options:
● -d <dbname> (--dbname=<dbname>)
● --disable-triggers

● -a (--data-only)
● -P function-name (args)

● -c (--clean) --function=function-name(args)

● -C (--create)
● -s (--schema-only)

● -i (--ignore-version)
● -S (--superuser=<username>)

● -I (--index=<index>)
● -v (--verbose)

● -l (--list)
● -t <table> (--table=<table>)

● -L <filename> (--use-list=<list-file>)
● -T <trigger> (--trigger=<trigger>)

● -n (--schema=<schema-name>)
● -F <format> (--format=<format>)
(c or t only)


pg_restore Examples

● $ pg_restore -a -F c -d prod2_db prod1_db.fc.dmp
Restores data only from a custom formatted file into database prod2_db

● $ pg_restore -c --schema=gold_partners -v -F t -d prod2_db prod_dump.tar.dmp
Cleans (removes data & structures first) then restores the gold_partners schema
From a tar formatted file into the prod2_db database

● $ pg_restore --schema-only -d qa1_db -F c prod1_db.fc.dmp
Restores the schema only (DDL) from a custom formatted file
into the qa1_db database


Create a TOC file from a dump

● $ pg_dump -Fc db1 > db1.fc.dmp
● $ pg_restore -Fc -l db1.dmp > db1.lst


Examining the TOC File
The TOC Header

; Archive created at Sat Mar 28 15:02:28 2009

; dbname: dbmon

; TOC Entries: 16

; Compression: 0

; Dump Version: 1.10-0

; Format: TAR

; Integer: 4 bytes

; Offset: 8 bytes

; Dumped from database version: 8.3.5

; Dumped by pg_dump version: 8.3.5

;


Examining the TOC File
The TOC File Contents

; Selected TOC Entries:

;

3; 2615 2200 SCHEMA - public postgres

1745; 0 0 COMMENT - SCHEMA public postgres

1746; 0 0 ACL - public postgres

1469; 1259 352554 TABLE public dbmon_thresh postgres

1467; 1259 352545 TABLE public vac_density_hist postgres

1468; 1259 352552 SEQUENCE public dbmon_thresh_dbmon_thresh_id_seq postgres

1747; 0 0 SEQUENCE OWNED BY public dbmon_thresh_dbmon_thresh_id_seq postgres

1748; 0 0 SEQUENCE SET public dbmon_thresh_dbmon_thresh_id_seq postgres

1736; 2604 352557 DEFAULT public dbmon_thresh_id postgres

1741; 0 352554 TABLE DATA public dbmon_thresh postgres

1740; 0 352545 TABLE DATA public vac_density_hist


Restore via a TOC File

● $ createdb dbmon2
● $ pg_restore -L db.lst -Ft dbmon-dg.dmp -d dbmon2


Restoring with psql

● $ pg_dump prod1_db > prod1_db.sql
● $ psql -ef prod1_db.sql > load_db.log 2>&1
● $ pg_dump prod1_db | psql -h qa_server
● $ pg_dumpall -g | psql -h dev_server -p 5433 > load_dev.log 2>&1
● $ pg_dump prod1_db | psql -e test_db > load_test_db.log 2>&1


Backup/Restore

Summary


PITR

● PostgreSQL has built-in facilities for Point in Time
Recovery
● PITR Backups
– Archiving the WAL segments
– Making Base Backups
● PITR Recovery
– Restore the last Base Backup
– Prepare the recovered system data directory
– Create a recovery.conf file
– Start the postmaster


PITR Backups
Enable WAL Archiving

● Enable / set the following parameters in the postgresql.conf file:
– archive_mode = on
– archive_command = 'cp %p /stage/wal/%f'
Can be any valid shell command (including scripts)
– archive_timeout = 0
● Special tags
– %p = full path (absolute path) and the filename of the WAL
segment to be archived
– %f = only the filename of the WAL segment
– %% = insert a % character in the command string.


PITR Backups
Enable WAL Archiving - Example

● mkdir /stage/wal
● chown postgres:postgres /stage/wal
(or other postgres cluster owner)

● Re-start the Server


PITR Backups
Create Transactions - Example

● Execute SQL commands / transactions
– Enable access, turn on applications, etc

● This should force the creation of multiple archived WAL files in
the /stage/wal directory

● WAL segments are copied when:
– The WAL segment is full (see checkpoint_segments)
– Number of seconds specified in archive_timeout has passed


PITR Backups
Create Base Backup - Example

● Execute pg_start_backup
– $ psql pitr_test
– # select pg_start_backup ('tag')
● Archive the cluster data directory (and any related tablespaces)
– $ tar -czvf /backups/pitr/<date>.data.tar.gz ./data
– $ rsync
– $ other copy methods
● Execute pg_stop_backup
– $ psql pitr_test
– # select pg_stop_backup ()


PITR Backups
Create More Transactions - Example

● Execute SQL commands / transactions
– The application, user connections, etc will continue to
generate transactions (and archived WAL segments)

● Verify the creation of additional archived WAL files in
the /stage/wal directory


PITR Recovery

(1) If available copy the original cluster data directory to
an alternate location
if space is an issue at least copy the old pg_xlog dir
it may contain additional unarchived WAL segments

(2) Ensure the postmaster is not running


PITR Recovery

If your backup was an rsync to a second server
then skip steps 3 & 4

(3) Remove the cluster data directory and any tablespace
directories

(4) Restore your last system backup
– make sure permissions are retained
– If you're using tablespaces then verify that the symbolic
links in pg_tblspc/ were restored


PITR Recovery

(5) Remove any wal segments from the pg_xlog dir that
were restored from the backup
– If you didn't backup pg_xlog then create it, make sure
you re-establish it as a symbolic link if needed (if you
had moved the tx logs to another disk)
– If needed also re-create the pg_xlog/archive_status
directory

(6) Copy the files from the original pg_xlog dir (if
available) into the new pg_slog dir (do a copy as
opposed to a move in case you need to start over).

PITR Recovery
(7) Create a recovery command file named recovery.conf
in the cluster data directory.
(8) [Optional] Temporarily modify pg_hba.conf to prevent
ordinary users from connecting until the recovery is
complete
(9) Start the server.
The server will go into recovery mode via the recovery.conf file.
Once the recovery is complete then the server will become
available and rename the recovery.conf file to recovery.done
If an error interrupts the recovery (or stops the server) then simply
re-starting the server will restart the recovery


PITR Recovery

(10) Verify the recovery.
If the database was not recovered properly (or to a state that you
desire) then go back to step 1
(11) restore the pg_hba.conf to its original state and run a
pg_ctl reload (if it was modified for the recovery)


PITR Recovery
recovery.conf

● Recovery settings are placed in the file 'recovery.conf'

● restore_command (string)
must return nonzero
– restore_command = 'cp /stage/wal/%f %p'
– restore_command = '/usr/local/bin/restore_shell.sh %p %f'


PITR Recovery
recovery.conf

● recovery_target_time (timestamp)
– specifies the time stamp up to which recovery will proceed.
– recovery_target_time and recovery_target_xid are mutually
exclusive
– The default is to recover to the end of the WAL log.


PITR Recovery
recovery.conf

● recovery_target_xid (string)
– specifies the transaction ID up to which recovery will proceed.
– Although transaction IDs are assigned sequentially at
transaction start, transactions can complete in a different
numeric order.
– All transactions committed before the specified XID will be
recovered


PITR Recovery
recovery.conf

● recovery_target_timeline (string)
– Specifies recovering into a particular timeline for complex re-
recovery scenarios. See the online docs for more info.

● log_restartpoints (boolean)
– If true, each restart point will be logged. This can be helpful to
track the progress of a long recovery. The default is false.


PITR Recovery

Summary


Warm Standby

● The PostgreSQL built-in facilities for PITR can be
leveraged to create a 'warm standby' server
● A Warm Standby server consists of the following:
– A server with a running postmaster (in recovery
mode)
– A recovery command that keeps the server in
recovery mode
– A continous stream of archived WAL segments
from the “master”
– A failover mechanism


Warm Standby
Overview

● Configure WAL archiving on the master
● Make the WAL segments available to the warm standby
server
● Setup recovered cluster on the warm standby server
● Setup the recovery command scripts
● Setup failover mechanism


Warm Standby
Setup WAL Archiving

● Enable WAL archiving in the postgresql.conf file (on the
master)
– archive_mode = on
– archive_command = 'cp %p /stage/wal/%f'
– archive_timeout = 0

● Create a directory for the archived WAL segments
– $ mkdir /stage/wal
– $ chmod postgres:postgres /stage/wal


Warm Standby
Make WAL segments available to the warm standby server

● NFS (not the best choice)
● rsync
● scp
● Other methods


Warm Standby
Setup recovered cluster on the warm standby server

Use the same method as described in the PITR
recovery steps


Warm Standby
Setup recovery command scripts

● Pseudocode for a suitable restore_command is:
triggered = false;
while (!NextWALFileReady() && !triggered)
{
sleep(100000L); /* wait for ~0.1 sec */
if (CheckForExternalTrigger())
triggered = true;
}
if (!triggered)
CopyWALFileForRecovery();
http://www.postgresql.org/docs/8.3/static/warm-standby.html


Warm Standby
Setup recovery command scripts – pg_standby

● PostgreSQL contrib pg_standby

* This program will be executed once in full for each file
* pg_standby.c
* requested by the warm standby server.
* Production-ready example of how to create a Warm Standby
*
* database server using continuous archiving as a
* It is designed to cater to a variety of needs, as well
* replication mechanism
* providing a customizable section.
*
*
* We separate the parameters for archive and next WAL file
* Original author:
* so that we can check the archive exists, even if the
Simon Riggs simon@2ndquadrant.com
* WAL file doesn't (yet).
* Current maintainer: Simon Riggs
*


Warm Standby
Setup recovery command scripts – recovery.conf file

● Using a shell script
– restore_command = '/home/postgres/wst/bin/shell_script.sh %p %f'

● Using the pg_standby contrib
– restore_command =
pg_standby [options] archivelocation %f %p %r [RESTARTWALFILE]
%r = “The size of the WAL archive can be minimized by using the %r option of the
restore_command. This option specifies the last archive file name that needs
to be kept to allow the recovery to restart correctly. This can be used to truncate
the archive once files are no longer required, if the archive is writable from the
standby server.” **

** http://www.postgresql.org/docs/8.3/static/warm-standby.html


Warm Standby

● pg_standby [options] **

Option Default Use cp or copy command to restore WAL files from archive.
-c yes Remove files from archivelocation so that no more than this many WAL files
before the current one are kept in the archive. Zero (the default) means not to
remove any files from archivelocation.
-d no Use ln command to restore WAL files from archive. Link is more efficient than
copy, but the default is copy since link will not work in all scenarios
-k <numfiles> 0 Remove files from archivelocation so that no more than this many WAL
files before the current one are kept in the archive. Zero (the default)
means not to remove any files from archivelocation. This parameter will
be silently ignored if restartwalfile is specified
-l no Use ln command to restore WAL files from archive. Link is more
efficient than copy, but the default is copy since link will not work in all
scenarios.

** http://www.postgresql.org/docs/8.3/static/pgstandby.html


Warm Standby

● pg_standby [options] **

Option Default Use cp or copy command to restore WAL files from archive.
-r <max retries> 3 Set the maximum number of times to retry the copy or link command if it
fails. After each failure, we wait for sleeptime * num_retries so that the
wait time increases progressively.
-s <sleep time> 5 Set the number of seconds (up to 60) to sleep between tests to see if
the WAL file to be restored is available in the archive yet.
-t <trigger file> none Specify a trigger file whose presence should cause recovery to end
whether or not the next WAL file is available.
-w <max wait time> 0 Set the maximum number of seconds to wait for the next WAL file, after
which recovery will end and the standby will come up. A setting of zero
(the default) means wait forever.

** http://www.postgresql.org/docs/8.3/static/pgstandby.html


Warm Standby

Summary


End


Backup and-recovery2

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