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PostgreSQL for beginners: Initial configuration

| by jpic | postgresql linux

Those are notes taken from the talk “PostgreSQL when it is not your job” by Christophe Pettus from PostgreSQL Experts Inc. at DjangoCon Europe 2012.

This article describes how to make a basic PostgreSQL configuration:

  • logging,
  • memory,
  • checkpoints,
  • planner.

Note: this article is mostly a transcript from the talk by Christophe Pettus: so send all the cookies to him. Thanks !

That's around 12 configuration options and you're done.

Logging

Starting with logging is a good idea because it's the best way of getting information for configuring the other settings.

  • be generous with logging; it's very low-impact on the system,
  • it's your best source of information for finding performance problems

Where to log

Three options:

  • syslog, if you have a syslog infrastructure you like already,
  • standard format to files - if you're using tools that need standard format,
  • otherwise, CSV format to files,

What to log ?

Christophe Pettus recommends this configuration, which you can copy and paste in your postgresql.conf:

log_destination = 'csvlog'
log_directory = 'pg_log
# enable file rotator
logging_collector = on
log_filename = 'postgres-%Y-%m-%d_%H%M%S'
log_rotation_age = 1d
log_retation_size = 1GB
# log statements that take more than 250ms
log_min_duration_statement = 250ms
log_checkpoints = on
log_connections = on
log_disconnections = on
log_lock_waits = on
log_temp_files = 0

Memory configuration

Three things you have to tweak:

  • shared_buffers,
  • work_mem,
  • maintenance_work_mem,

shared_buffers

  • below 2GB of RAM, set to 20% of total system memory,
  • below 32GB, set to 25% of total system memory,
  • avec 32GB, set to 8GB,

work_mem

  • start low: 32-64MB,
  • then wait until there is some server load (simulated or real),
  • look for ‘temporary file’ lines in logs,
  • it will say “I'm creating temporary file of this size”,
  • set work_mem to 2-3x the largest temp file you see.

This can be a huge spped-up if set properly ! This is probably the setting that has the most impact on postgresql performance !

But if you find yourself setting this super high like 8GB, you probably don't want to set it that high. Because every time someone does an operation in postgres like a SORT, it could potentially use that amount of memory per planner.

If you're having problems of memory exhaustion problems, this is the best setting to back off.

maintenance_work_meme

This is the amount of memory that PostgreSQL uses for indexing or vacuum operations.

  • 10% of system memory, up to 1GB,
  • Maybe even higher if you are having VACUUM problems,

effective_cache_size

Unlike other parameters we've seen, this one won't actually allocate any memory.

  • set to the amount of file system cache available,
  • if you don't know, set it to 50% of total system memory,

Checkpoints

  • A checkpoint is a complete flush of dirty buffers to disk.
  • Potentially a lot of I/O (Input/Output),
  • Done when the first of two thresholds are hit:
    • A particular number of WAL (Write-Ahead-Log) segments have been written … basically you don't have to worry about that, it's when a certain amount of database activity has happened.
    • A timeout occurs.

Settings part I

# that's the right value according to Chripstophe
wal_buffers = 16MB
checkpoint_completion_target = 0.9
checkpoint_timeout = 10m-30m, # Depends on the restart time

The trade off about checkpoint_timeout is: the higher it goes, you'll generally get better performance, but the longer it will take postgresql to restart after a crash.

Settings part II

  • Look for checkpoint entries in the logs
  • Is it happening more often than checkpoint_timeout ?
  • Then adjust checkpoint_segments so that checkpoints happen due to timeouts rather than filling segments,

Settings part III, warnings

  • The WAL can take up to 316MBcheckpoint_segments on disk, so the higher that number goes the more space on disk it will be using … but on modern disks it's not really that much disk,
  • Restarting PostgreSQL can take up to checkpoint_timeout (but usually less, like 20% of that),

Planner settings

  • effective_io_concurrency, set to the number of I/O channels; otherwise ignore it. For example if you have a stack of hard drives or an SSD drive that has 32 channels then set it to the number of channels.
  • random_page_cost, 3.0 for a typical RAID10 array, 2.0 for a SAN, 1.1 for Amazon EBS, else don't tweak it.