File creation dates of archived WAL files not in line with archive_timeout in PostgreSQL 9.6

Question

I have configured archive_timeout = 5 (i.e. 5 seconds) for a PostgreSQL 9.6 database in order to conduct tests around point-in-time recovery. My archive_command invokes cp into a local archive directory.

If I check the file modification dates in that local archive directory I see that WAL files are typically created in there only approx. every 20 seconds apart, i.e. far more than the configured 5 seconds. This is for PostgreSQL 9.6 running inside a Docker container on a lightly loaded system.

What could explain this large discrepancy?

Answer 1

The documentation has an answer to this:

When this parameter is greater than zero, the server will switch to a new segment file whenever this many seconds have elapsed since the last segment file switch, and there has been any database activity, including a single checkpoint (checkpoints are skipped if there is no database activity).

Based on your observation and the above quote, I guess you have a checkpoint every 20 seconds or so, and a very low rate of changes.