Overview of Postgres Utility Processes

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Overview of Postgres Utilities Processes

Dave Thomas | 8.22.13

Techie Topic Series


•  Overview – What are all these processes? •  Logger Process •  Checkpointer Process

•  Writer Process •  WAL Writer Process •  Autovacuum Launcher Process

•  Stats Collector Process •  Streaming Replication Process •  Resources

Presentation Agenda


What are all these processes?

•  PostgreSQL utilizes separate processes instead of threads, which results in the above ps output for a freshly started cluster

•  Each connection to the cluster will cause the postmaster to spawn a process which may look like the following:

•  The above shows a connection from 127.0.0.1 to the test database by the user Postgres.


•  Logging is an optional process, the default is OFF

•  All the utility processes + user backends + postmaster daemon attach to the logger process

•  All process information is logged under $PGDATA/pg_log in the file name specified by log_filename −  If the data directory is created with INITDB command, then the pg_log

directory will need to be created manually. −  You can also log to syslog and CSV formatted files

•  Increasing the amount of logging will increase the I/O load on the server.

•  http://www.enterprisedb.com/docs/en/9.2/pg/runtime-config-logging.html

Logger Process


•  This process is responsible for flushing all dirty data pages to disk (called a checkpoint) −  This process can cause a significant I/O load on the server.

•  Checkpoints are performed every checkpoint_timeout seconds or when all checkpoint_segments are filled, whichever comes first.

•  More frequent checkpoints will provide for faster after-crash recovery, but this must be balanced by the increased I/O load generated.

•  http://www.enterprisedb.com/docs/en/9.2/pg/wal-configuration.html

Checkpointer Process


•  Also known as BGWRITER process and is a mandatory process.

•  Wakes every bgwriter_delay seconds and searches through the shared buffer pool looking for modified pages, writes them to disk, and evicts those pages from the shared buffer pool.

•  No more than bgwriter_lru_maxpages buffers will be written by the background writer in a given pass.

•  http://www.enterprisedb.com/docs/en/9.2/pg/runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER

Writer Process


•  This is a mandatory process

•  Writes and syncs Write Ahead Logs(WAL) to disk upon transaction commit

•  WALs allow transactions to be recovered in the event of a system crash

•  http://www.enterprisedb.com/docs/en/9.2/pg/wal-configuration.html

WAL Writer Process


•  Autovacuuming is an optional process, default is ON.

•  Automates the execution of VACUUM and ANALYZE commands

•  A daemon which spawns autovacuum worker processes for each database −  Starts one worker on each database (up to autovacuum_max_workers)

every autovacuum_naptime seconds −  Worker processes check each table and index in a database and

executes VACUUM or ANALYZE as needed.

•  http://www.enterprisedb.com/docs/en/9.2/pg/runtime-config-autovacuum.html

Autovacuum Launcher Process


•  Stats collector process is an optional process, default is ON.

•  Collects information about cluster activity. −  Number of access to the tables and indexes −  Total number of rows in each table −  Information about VACUUM and ANALYZE actions for each table.

•  Collection of statistics adds some overhead to query execution, but allows the query planner to make better choices.

•  http://www.enterprisedb.com/docs/en/9.2/pg/monitoring-stats.html

Stats Collector Process


•  WAL Sender

−  This process sends WALs the standby server. −  One process for each standby server.

•  WAL Receiver

−  This process receives WALs from the the primary server. −  The WALs are then replayed to bring the standby in sync with the primary.

•  http://www.enterprisedb.com/docs/en/9.2/pg/warm-standby.html#STREAMING-REPLICATION

Streaming Replication Processes


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Additional Resources- on EnterpriseDB.com


Overview of Postgres Utility Processes

Technology

optional process

process information

bgwriter process

mandatory process

writer writer process

autovacuum worker processes

utility processes

separate processes