初步学习pg_control文件之十五

接前文  初步学习pg_control文件之十四

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 = ;
int            MaxBackends = ;
int            MaxConnections = ;

/*
 * 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);
        ...
 
    }
}