The log_flush function guarantees that all log records whose
DB_LSN values
are less than or equal to the lsn argument have been
written to disk. If lsn is NULL, all records in the
log are flushed.
The log_flush function returns a non-zero error value on failure and 0 on success.
Errors
The log_flush function may fail and return a non-zero error for the following conditions:
EINVAL
An invalid flag value or parameter was specified.
The log_flush function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions.
If a catastrophic error has occurred, the log_flush function may fail and return
DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail
in the same way.