The Essential
postgresql.conf
Robert Treat
Beijing Perl Workshop, China, 2008

Introduction
Robert Treat
DBA, Postgres Major Contributor
http://www.xzilla.net/writings.html
OmniTI
Internet Scalability Experts
http://omniti.com/is/hiring

The Grand Scheme
Understanding the postgresql.conf
Essential postgresql.conf parameters

The Grand Scheme
Understanding the postgresql.conf
Essential postgresql.conf parameters

Understanding the postgresql.conf
Types of settings
When settings take effect
postgresql.conf gotchas
Viewing your current settings

Types of Settings
● Boolean : true, false, on, off
● Integer : whole numbers (2112)
● Float : decimal values (21.12)
● Memory/Disk Units : 128MB, 2112GB
● Avoid using integers to size memory params
● Time : d,m,s (30s)
● Strings : single quotes ('pg_log')
● Enum : single quotes ('WARNING' or 'ERROR')
● Limited set of acceptable strings
● Lists : comma separated ('”$user”,public,tsearch2')

When Settings Take Effect
● Internal : set at compile time
● Postmaster : requires database restart
● Backend : must be set before session start
● Sighup : change by reloading conf file
●
pg_reload_conf(), pg_ctl reload, kill -HUP
● Superuser : set at runtime by superuser
● User : set at runtime, effects only single session
See pg_settings.context to determine setting type

When Settings Take Effect
● Internal : set at compile time
● Postmaster : requires database restart
● Backend : must be set before session start
● Sighup : change by reloading conf file
●
pg_reload_conf(), pg_ctl reload, kill -HUP
● Superuser : set at runtime by superuser
● User : set at runtime, effects only single session
See pg_settings.context to determine setting type

postgresql.conf Gotchas!
● $PGDATA/postgresql.conf
● Watch out for symbolic links
● Some distros put it other places (/etc)
● Lines with #
● Line is a comment, has no effect
● On fresh install, uses default
● On <8.3, comment a line does not restore the default
● Settings listed multiple times
● Last one wins

View Your Current Settings
● Look in postgresql.conf
● Works if you follow best practice
● Not definitive!
● Show all, show <setting>
● Shows current value
● Watch out for session specific changes
● Select * from pg_settings
● Shows current value
●
“source” column shows if session specific

Essential postgresql.conf parameters
Getting connected
Logging
Sizing Memory
WAL Management
Checkpoint Management
Vacuuming
Leftovers

Getting Connected
● listen_addresses
● Requires restart!
● Default is localhost only
● Use * to enable tcp/ip
● max_connections
● Requires restart!
● 100 default (apache children default is higher)
● Affects other settings (work_mem)
● For 1000+ connections, use connection pool

Logging
● log_destination, log_directory, log_filename
● Find out where you are logging
● Might be good to log to different disk than data
● log_min_error_statement
● Make sure set to at least 'ERROR'
● log_line_prefix
● Generic recommendation: '%t:%r:%u@%d:[%p]: '
● Timestamp, connecting host, username, database, pid

Sizing Memory
● shared_buffers
● Requires Restart!
● Most important setting for good performance
● Allocation of memory for Postgres caching
● ~ 20% of Total RAM (up to 2GB)
● 32GB of RAM, 1920MB probably ok
● Windows users probably want less
● May require adjusting kernel params (shmmax)
● http://www.postgresql.org/docs/current/interactive/kernel-resources.html

Sizing Memory
● effective_cache_size
● Most important setting for good performance
● Based on memory available for filesystem cache
● SET = TRAM – (DRAM – PGRAM)
● TRAM = Total RAM
● DRAM = RAM needed by OS and other applications
● PGRAM = RAM needed for Postgres (shared_buffers, etc...)
● Guide for available memory, not an allocation

Sizing Memory
● work_mem
● Used by queries for sorting
● Higher values for more complex queries
● Limit is Per sort
● 5 sorts in 1 query = 5*work_mem
● maintenance_work_mem
● Used by vacuum, indexing, and similar operations
● Semi-Allocation of memory when needed
● Values up to 128MB have been found useful

WAL Management
● wal_buffers
● Requires restart!
● needed for write-heavy databases
● 1024kB is good starting point
● wal_sync_method
● WAL is sync'd after every transaction
● fsync is ok default, but tuning can be helpful sometimes
● http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm
● synchronous_commit
● Can increase tps by large margin
● Allows data loss without corruption
● BAD: fsync=off
● GOOD: synchronous_commit=off
● Set per database, user, or transaction

Checkpoint Management
● checkpoint_segments
● Controls amount of data required per checkpoint
● Recommend default of 10
● Set higher for heavy write activity
● 30-300 is not uncommon
● Increases space needed in pg_xlog
● Increases recovery time after crash
● checkpoint_completion_target
● With checkpoint_segments >= 10, set this to .9
● http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm

Vacuum Management
● autovacuum
● Just do it! (Default on 8.3+)
● default_statistics_target
● Defines amount of data collected on tables for planner
● Important for good query planning
● 100 is a better default (might need further tuning)

Vacuum Management
● max_fsm_relations, max_fsm_pages
● Both Require Restart!
● Track dead tuples in database cluster
● fsm_relations – number of tables in all databases
● fsm_pages – number of pages with dead tuples in all databases
● “vacuum verbose” in postgres database
INFO: free space map contains 5293 pages in 214 relations
DETAIL: A total of 8528 page slots are in use (including overhead).
8528 page slots are required to track all free space.
Current limits are: 204800 page slots, 1000 relations, using 1265 kB
● check_postgres script has a check for both
● http://bucardo.org/check_postgres/
● Requires pg_freespacemap from contrib
● These go away in Postgres 8.4 (yay!)

Leftovers
● random_page_cost
● For server grade hardware, most set to 3
● max_prepared_transactions
● Can be set to zero if not using prepared transactions
● constraint_exclusion
● Needed for table partitioning
● Good for setting at database or query level

The End
● Additional Resources
● http://www.postgresql.org/docs/8.3/interactive/runtime-config.html
● http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server
● Slides
● http://www.xzilla.net/writings.html
● Xìe xìe