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.
Basically according to the manual ( http://www.postgresql.org/docs/9.1/static/runtime-config-wal.html), I know: "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 -- Tianyin XU, http://cseweb.ucsd.edu/~tixu/
