Hi all,
I've been investigating an issue with our PostgreSQL 9.1.1 (Linux x86-64 CentOS 5.8) database where restartpoints suddenly stop being generated on the slave after working correctly for a week or two. The symptom of the problem is that the pg_xlog directory on the slave doesn't get cleaned up, and the log_checkpoints output (eg. restartpoint starting: time) stops appearing.
I was able to extract a core dump of the bgwriter process while it was in BgWriterNap. I inspected ckpt_start_time and last_checkpoint_time; ckpt_start_time was 1345578533 (... 19:48:53 GMT) and last_checkpoint_time was 1345578248 (... 19:44:08 GMT). Based upon these values, I concluded that it's performing checkpoints but missing the "if (ckpt_performed)" condition (ie. CreateRestartPoint returns false); it's then setting last_checkpoint_time to now - 5 minutes + 15 seconds.
There seems to be two causes of a false retval in CreateRestartPoint; the first is if !RecoveryInProgress(), and the second is if "the last checkpoint record we've replayed is already our last restartpoint". The first condition doesn't seem likely; does anyone know how we might be hitting the second condition? We have continuous traffic on the master server in the range of 1000 txn/s, and the slave seems to be completely up-to-date, so I don't understand how we could be hitting this condition.
Mathieu