PostgreSQL is a popular open source relational database management system that is widely used for storing and managing data. One of the common issues that can be encountered in PostgreSQL is replication lag.
In this blog, we will discuss what replication lag is, why it occurs, and how to mitigate it in PostgreSQL.
What is replication lag?
Replication lag is the delay between the time when data is written to the primary database and the time when it is replicated to the standby databases. In PostgreSQL, replication lag can occur due to various reasons such as network latency, slow disk I/O, long-running transactions, etc.
Replication lag can have serious consequences in high-availability systems where standby databases are used for failover. If the replication lag is too high, it can result in data loss when failover occurs.
The most common approach is to run a query referencing this view in the primary node.
1 2 3 4 5 6 7 | postgres=# SELECT pid,application_name,client_addr,client_hostname,state,sync_state,replay_lag postgres-# FROM pg_stat_replication postgres-# ; pid | application_name | client_addr | client_hostname | state | sync_state | replay_lag -----+------------------+-------------+-----------------+-------+------------+------------ (0 rows) postgres=# |
Queries to check in the Standby node:
1 2 3 4 5 6 | postgres=# select pg_is_in_recovery(),pg_is_wal_replay_paused(), pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn(), pg_last_xact_replay_timestamp(); pg_is_in_recovery | pg_is_wal_replay_paused | pg_last_wal_receive_lsn | pg_last_wal_replay_lsn | pg_last_xact_replay_timestamp -------------------+-------------------------+-------------------------+------------------------+------------------------------- t | f | D1/8000000 | D1/8000000 | 2023-03-16 11:37:57.861711+00 (1 row) postgres=# |
Why does replication lag occur?
Replication lag can occur due to various reasons, such as:
Network latency: Network latency is the delay caused by the time it takes for data to travel between the primary and standby databases.
Various factors, such as the distance between the databases, network congestion, etc., can cause this delay:
Slow disk I/O: Slow disk I/O can be caused by various factors such as disk fragmentation, insufficient disk space, etc. Slow disk I/O can delay writing data to the standby databases.
Long-running transactions: Long-running transactions can cause replication lag because the changes made by these transactions are not replicated until the transaction is committed.
A poor configuration, like setting low numbers of max_wal_senders while processing huge numbers of transaction requests.
Sometimes the server recycles old WAL segments before the backup can finish and cannot find the WAL segment from the primary.
Usually, this is also due to the checkpointing behavior where WAL segments are rotated or recycled.
Mitigating replication lag in PostgreSQL
There are several ways to mitigate replication lag in PostgreSQL, such as:
Increasing the network bandwidth: Increasing the network bandwidth between the primary and standby databases can help reduce replication lag caused by network latency.
Using asynchronous replication: Asynchronous replication can help reduce replication lag by allowing the standby databases to lag behind the primary database.
This means that the standby databases do not have to wait for the primary database to commit transactions before replicating the data.
Tuning PostgreSQL configuration parameters: Tuning the PostgreSQL configuration parameters such as wal_buffers, max_wal_senders, etc.
can help improve replication performance and reduce replication lag.
Monitoring replication lag: Monitoring replication lag can help identify the cause of the lag and take appropriate actions to mitigate it.
PostgreSQL provides several tools, such as pg_stat_replication, pg_wal_receiver_stats, etc., for monitoring replication lag.
Conclusion
Replication lag is a common issue in PostgreSQL that can seriously affect high-availability systems.
Understanding the causes of replication lag and taking appropriate measures to mitigate it can help ensure the availability and reliability of the database system.
By increasing network bandwidth, using asynchronous replication, tuning PostgreSQL configuration parameters, and monitoring replication lag, administrators can mitigate replication lag and ensure a more stable and reliable database environment.
Percona Distribution for PostgreSQL provides the best and most critical enterprise components from the open-source community, in a single distribution, designed and tested to work together.
I think it’s fare to explain briefly the consequences of asynchronous replication. It’s arguable that generating sporadic lag will reduce general replication lag. Another important parameters to consider in wal writes are checkpoint_timeout and also min and max_wal_size.