Question: What is commit_delay in PostgreSQL and how is it used?
Answer
commit_delay
is a configuration parameter in PostgreSQL that can be used to improve performance under certain conditions, especially in systems experiencing high contention due to concurrent transactions committing at the same time. By introducing a delay before the final commit record is written to the disk, commit_delay
aims to reduce this contention.
How Does commit_delay
Work?
In PostgreSQL, when a transaction commits, it needs to write a commit record to the Write-Ahead Logging (WAL). This process can lead to disk I/O contention if many transactions are committing simultaneously, as each transaction tries to write to the WAL. The commit_delay
setting specifies a delay period (in microseconds) that the server waits after generating the WAL for a commit and before actually writing it to the disk. This delay allows multiple commit records to be combined into fewer disk operations, thus reducing I/O contention and potentially increasing throughput.
Usage
The commit_delay
parameter is often used in conjunction with another parameter called commit_siblings
. The commit_siblings
parameter specifies the minimum number of concurrent transactions needed to trigger the commit_delay
. For example, if commit_siblings
is set to 5, the commit_delay
will only be applied if there are at least five transactions committing at the same time.
Here is how you might set these parameters in the postgresql.conf
file or dynamically using the ALTER SYSTEM
command:
-- Set commit_delay to 10 milliseconds (10000 microseconds) ALTER SYSTEM SET commit_delay = 10000; -- Set commit_siblings to 5 ALTER SYSTEM SET commit_siblings = 5; -- Reload configuration to apply changes SELECT pg_reload_conf();
Considerations
While commit_delay
can improve throughput by reducing I/O contention, it can also increase the latency of individual commit operations because of the added delay. Therefore, it's important to test this setting thoroughly in a development or staging environment before applying it in production. Ideally, it should be used in high-load environments where disk I/O contention is a significant bottleneck.
In conclusion, commit_delay
and commit_siblings
are tools in PostgreSQL that can help manage disk I/O contention during periods of high transactional activity. Proper tuning of these parameters based on specific workload characteristics can lead to improved system performance.
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Other Common PostgreSQL Questions (and Answers)
- How can I limit the number of rows updated in a PostgreSQL query?
- How do you limit the number of rows deleted in PostgreSQL?
- How do you use the PARTITION OVER clause in PostgreSQL?
- What are PostgreSQL replication slots and how do they work?
- How can you partition an existing table in PostgreSQL?
- How do you partition a table by multiple columns in PostgreSQL?
- How do you check the replication status in PostgreSQL?
- What are the scaling limits of PostgreSQL?
- How do you scale Azure PostgreSQL?
- How can I improve delete performance in PostgreSQL?
- How can PostgreSQL be auto-scaled?
- What are the best practices for PostgreSQL replication?
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