Hi, pgsql-docs,
I was confused by the system behavior of PG's write ahead log, specifically the relation between commit_delay, commit_siblings, and fsync.
"commit_delay" specifies the delay before a transaction attempts to flush the WAL buffer out to disk.
"commit_siblings" specifies minimum number of concurrent open transactions to require before performing the commit_delay delay.
However, after looking at the code, I figured out these configuration parameters are also controlled by fsync.
The only usage of the two parameter I can find is:
/* src/backend/access/transam/xact.c */
static TransactionId
RecordTransactionCommit(void)
...
if (CommitDelay > 0 &&
enableFsync &&
MinimumActiveBackends(CommitSiblings))
pg_usleep(CommitDelay);
...
}
where, CommitDelay stores the parameter of "commit_delay", CommitSiblings stores the parameter of "commit_siblings", and enableFsync stores the parameter of "fsync".
It seems the two parameters (commit_delay and commit_siblings) only have effect when fsync is set to be *on*.
But, unfortunately, such behavior is not covered in the manual. The mistake I made is only set the two values but disabling fsync so the expected PG does not behave as I expected. I think it's definitely helpful to specify the relations between these configuration parameters either in the manual, or in the source code by giving warnings logs.
Thanks a lot!
Tianyin
