postgresql/contrib/pg_autovacuum/README.pg_autovacuum

pg_autovacuum README
--------------------

pg_autovacuum is a libpq client program that monitors all the
databases associated with a PostgreSQL server.  It uses the statistics
collector to monitor insert, update and delete activity.  

When a table exceeds a insert or delete threshold (for more detail on
thresholds, see "Vacuum and Analyze" below) then that table will be
vacuumed and/or analyzed.

This allows PostgreSQL to keep the FSM (Free Space Map) and table
statistics up to date, and eliminates the need to schedule periodic
vacuums.

The primary benefit of pg_autovacuum is that the FSM and table
statistic information are updated more nearly as frequently as needed.
When a table is actively changing, pg_autovacuum will perform the
VACUUMs and ANALYZEs that such a table needs, whereas if a table
remains static, no cycles will be wasted performing this
unnecessarily.

A secondary benefit of pg_autovacuum is that it ensures that a
database wide vacuum is performed prior to XID wraparound.  This is an
important, if rare, problem, as failing to do so can result in major
data loss.  (See the section in the _Administrator's Guide_ entitled
"Preventing transaction ID wraparound failures" for more details.)

KNOWN ISSUES:
-------------

pg_autovacuum has been tested under Redhat Linux (by me) and Debian
GNU/Linux, Solaris, and AIX (by Christopher B. Browne) and all known
bugs have been resolved.  Please report any problems to the hackers
list.

pg_autovacuum requires that the statistics system be enabled and
reporting row level stats.  The overhead of the stats system has been
shown to have a significant cost under certain workloads.  For
instance, a tight loop of queries performing "select 1" was found to
run nearly 30% slower when stats were enabled.  However, in practice,
with more realistic workloads, the stats system overhead is usually
nominal.

pg_autovacuum does not get started automatically by either the
postmaster or by pg_ctl.  Similarly, when the postmaster exits, no one
tells pg_autovacuum.  The result of that is that at the start of the
next loop, pg_autovacuum will fail to connect to the server and
exit().  Any time it fails to connect pg_autovacuum exit()s.

While pg_autovacuum can manage vacuums for as many databases as you
may have tied to a particular PostgreSQL postmaster, it can only
connect to a single PostgreSQL postmaster.  Thus, if you have multiple
postmasters on a particular host, you will need multiple pg_autovacuum
instances, and they have no way, at present, to coordinate between one
another to ensure that they do not concurrently vacuum big tables.

TODO:
-----

At present, there are no sample scripts to automatically start up
pg_autovacuum along with the database.  It would be desirable to have
a SysV script to start up pg_autovacuum after PostgreSQL has been
started.

Some users have expressed interest in making pg_autovacuum more
configurable so that certain tables known to be inactive could be
excluded from being vacuumed.  It would probably make sense to
introduce this sort of functionality by providing arguments to specify
the database and schema in which to find a configuration table.

It would also be desirable for the daemon to monitor how busy the
system is, with a view to deferring vacuums until there is less other
activity.

INSTALL:
--------

As of postgresql v7.4 pg_autovacuum is included in the main source
tree under contrib.  Therefore you merely need to "make && make
install" (similar to most other contrib modules) and it will be
installed for you.

If you are using an earlier version of PostgreSQL, uncompress the
tar.gz file into the contrib directory and modify the contrib/Makefile
to include the pg_autovacuum directory.  pg_autovacuum will then be
built as part of the standard postgresql install.  It is known to work
with v7.3 releases; it is not presently compatible with v7.2.

make sure that the following are set in postgresql.conf:

  stats_start_collector = true
  stats_row_level = true

Start up the postmaster, then execute the pg_autovacuum executable.

If you have a script that automatically starts up the PostgreSQL
instance, you might add in, after that, something similar to the
following:

  sleep 10    # To give the database some time to start up
  $PGBINS/pg_autovacuum -D -s $SBASE -S $SSCALE ... [other arguments]

Command line arguments:
-----------------------

pg_autovacuum has the following optional arguments:

-d debug: 0 silent, 1 basic info, 2 more debug info,  etc...
-D daemonize: Detach from tty and run in background.
-s sleep base value: see "Sleeping" below.
-S sleep scaling factor: see "Sleeping" below.
-v vacuum base threshold: see Vacuum and Analyze.
-V vacuum scaling factor: see Vacuum and Analyze.
-a analyze base threshold: see Vacuum and Analyze.
-A analyze scaling factor: see Vacuum and Analyze.
-L log file: Name of file to which output is submitted, otherwise STDERR
-U username: Username pg_autovacuum will use to connect with, if not
   specified the current username is used.
-P password: Password pg_autovacuum will use to connect with.
-H host: host name or IP to connect to.
-p port: port used for connection.
-h help: list of command line options.

Numerous arguments have default values defined in pg_autovacuum.h.  At
the time of writing they are:

-d 1
-v 1000
-V 2   
-a 500 (half of -v if not specified)
-A 1   (half of -v if not specified)
-s 300 (5 minutes)
-S 2


Vacuum and Analyze:
-------------------

pg_autovacuum performs either a VACUUM ANALYZE or just ANALYZE
depending on the mixture of table activity (insert, update, or
delete):

- If the number of (inserts + updates + deletes) > AnalyzeThreshold, then
  only an analyze is performed.

- If the number of (deletes + updates) > VacuumThreshold, then a
  vacuum analyze is performed.

VacuumThreshold is equal to:
    vacuum_base_value + (vacuum_scaling_factor * "number of tuples in the table")

AnalyzeThreshold is equal to:
    analyze_base_value + (analyze_scaling_factor * "number of tuples in the table")

The AnalyzeThreshold defaults to half of the VacuumThreshold since it
represents a much less expensive operation (approx 5%-10% of vacuum),
and running ANALYZE more often should not substantially degrade system
performance.

Sleeping:
---------

pg_autovacuum sleeps for a while after it is done checking all the
databases.  It does this in order to limit the amount of system
resources it consumes.  This allows the system administrator to
configure pg_autovacuum to be more or less aggressive.

Reducing the sleep time will cause pg_autovacuum to respond more
quickly to changes, whether they be database addition/removal, table
addition/removal, or just normal table activity.

On the other hand, setting pg_autovacuum to sleep values too
aggressively (to too short periods of time) can have a negative effect
on server performance.  For instance, if a table gets vacuumed 5 times
during the course of a large set of updates, this is likely to take a
lot more work than if the table was vacuumed just once, at the end.

The total time it sleeps is equal to:

  base_sleep_value + sleep_scaling_factor * "duration of the previous
  loop"

Note that timing measurements are made in seconds; specifying
"pg_vacuum -s 1" means pg_autovacuum could poll the database up to 60
times minute.  In a system with large tables where vacuums may run for
several minutes, rather longer times between vacuums are likely to be
appropriate.

What pg_autovacuum monitors:
----------------------------

pg_autovacuum dynamically generates a list of all databases and tables
that exist on the server.  It will dynamically add and remove
databases and tables that are removed from the database server while
pg_autovacuum is running.  Overhead is fairly small per object.  For
example: 10 databases with 10 tables each appears to less than 10k of
memory on my Linux box.