The Essential postgresql.conf

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

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

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

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The Essential postgresql.conf

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