PostgreSQL 15 Advanced DBA Series: Troubleshooting PostgreSQL 15 replication lag with IO concurrency 

There are several ways to reduce replication lag with IO concurrency in PostgreSQL 15:

1. Increase the number of parallel workers: PostgreSQL 15 allows you to increase the number of parallel workers for a specific query, which can reduce replication lag by allowing the query to be executed more quickly. You can increase the number of parallel workers for a specific query by setting the max_parallel_workers_per_gather configuration parameter.
2. Increase the number of parallel queries: PostgreSQL 15 allows you to increase the number of parallel queries that can be executed simultaneously, which can reduce replication lag by allowing more queries to be executed in parallel. You can increase the number of parallel queries by setting the max_parallel_workers configuration parameter.
3. Increase the IO concurrency: By increasing the IO concurrency, you can reduce replication lag by allowing the disk to service more requests at the same time. You can increase the IO concurrency by setting the max_wal_senders configuration parameter, which controls the number of WAL (write-ahead log) senders that can be active at the same time.
4. Use SSDs: Using solid-state drives (SSDs) instead of traditional hard drives can also help to reduce replication lag by increasing the disk’s IO performance.
5. Use Write-Ahead Logging (WAL) compression: Using WAL compression can reduce the amount of data that needs to be written to the WAL, which can reduce replication lag by allowing the WAL to be written more quickly.
6. Use Streaming Replication: Streaming replication allows the slave to stay as close to the master as possible by streaming the WAL directly to the slave, reducing the time it takes for changes to be propagated to the slave.
7. Use pg_rewind: pg_rewind is a tool that allows you to synchronize a standby server with the primary, by rewinding the standby to a specific point in time and applying the missing changes.
8. Monitor replication lag: Continuously monitor replication lag by using tools like pg_stat_replication, which gives you a clear picture of how much lag is present.

It is worth noting that it’s a good practice to test the performance of these changes before applying them in a production environment. Also, you should always have a backup of your data before attempting any changes to your database.