再看如下这个：
int MaxConnections;
应该说，它是一个参考值，在global.c中有如下定义
/* * Primary determinants of sizes of shared-memory structures. MaxBackends is * MaxConnections + autovacuum_max_workers + 1 (it is computed by the GUC * assign hooks for those variables): */ int NBuffers = 1000; int MaxBackends = 100; int MaxConnections = 90;
/* * This must be called ONCE during postmaster or standalone-backend startup */ void StartupXLOG(void) { … /* * If any of the critical GUCs have changed, log them before we allow * backends to write WAL. */ LocalSetXLogInsertAllowed(); XLogReportParameters(); … }
/* * Check if any of the GUC parameters that are critical for hot standby * have changed, and update the value in pg_control file if necessary. */ static void XLogReportParameters(void) { if (wal_level != ControlFile->wal_level || MaxConnections != ControlFile->MaxConnections || max_prepared_xacts != ControlFile->max_prepared_xacts || max_locks_per_xact != ControlFile->max_locks_per_xact) { … ControlFile->MaxConnections = MaxConnections; ControlFile->max_prepared_xacts = max_prepared_xacts; ControlFile->max_locks_per_xact = max_locks_per_xact; ControlFile->wal_level = wal_level; UpdateControlFile(); } }
它就是一个参考值：
/* * This must be called ONCE during postmaster or standalone-backend startup */ void StartupXLOG(void) { … /* REDO */ if (InRecovery) { … /* Check that the GUCs used to generate the WAL allow recovery */ CheckRequiredParameterValues(); … if (record != NULL) { … /* * main redo apply loop */ do { … /* * If we are attempting to enter Hot Standby mode, process * XIDs we see */ if (standbyState >= STANDBY_INITIALIZED && TransactionIdIsValid(record->xl_xid)) RecordKnownAssignedTransactionIds(record->xl_xid); RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record); … } while (record != NULL && recoveryContinue); … } … } … }
/* * XLOG resource manager's routines * * Definitions of info values are in include/catalog/pg_control.h, though * not all record types are related to control file updates. */ void xlog_redo(XLogRecPtr lsn, XLogRecord *record) { … if (info == XLOG_NEXTOID) { … } … else if (info == XLOG_PARAMETER_CHANGE) { … /* Check to see if any changes to max_connections give problems */ CheckRequiredParameterValues(); } }
再看下面：
/* * Check to see if required parameters are set high enough on this server * for various aspects of recovery operation. */ static void CheckRequiredParameterValues(void) { ... if (InArchiveRecovery && EnableHotStandby) { .../* We ignore autovacuum_max_workers when we make this test. */ RecoveryRequiresIntParameter("max_connections", MaxConnections, ControlFile->MaxConnections);
...
} }