接前文：

继续学习，pg_control文件在何处形成的？是在initdb的时候，运用的函数如下：

/* * This func must be called ONCE on system install.  It creates pg_control * and the initial XLOG segment. */voidBootStrapXLOG(void){    CheckPoint    checkPoint;    char       *buffer;    XLogPageHeader page;    XLogLongPageHeader longpage;    XLogRecord *record;    bool        use_existent;    uint64        sysidentifier;    struct timeval tv;    pg_crc32    crc;    /*     * Select a hopefully-unique system identifier code for this installation.     * We use the result of gettimeofday(), including the fractional seconds     * field, as being about as unique as we can easily get.  (Think not to     * use random(), since it hasn't been seeded and there's no portable way     * to seed it other than the system clock value...)  The upper half of the     * uint64 value is just the tv_sec part, while the lower half is the XOR     * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness     * unnecessarily if "uint64" is really only 32 bits wide.  A person     * knowing this encoding can determine the initialization time of the     * installation, which could perhaps be useful sometimes.     */    gettimeofday(&tv, NULL);    sysidentifier = ((uint64) tv.tv_sec) << 32;    sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);    ...    /* Now create pg_control */    memset(ControlFile, 0, sizeof(ControlFileData));    /* Initialize pg_control status fields */    ControlFile->system_identifier = sysidentifier;    ControlFile->state = DB_SHUTDOWNED;    ControlFile->time = checkPoint.time;    ControlFile->checkPoint = checkPoint.redo;    ControlFile->checkPointCopy = checkPoint;    /* Set important parameter values for use when replaying WAL */    ControlFile->MaxConnections = MaxConnections;    ControlFile->max_prepared_xacts = max_prepared_xacts;    ControlFile->max_locks_per_xact = max_locks_per_xact;    ControlFile->wal_level = wal_level;    /* some additional ControlFile fields are set in WriteControlFile() */    WriteControlFile();    /* Bootstrap the commit log, too */    BootStrapCLOG();    BootStrapSUBTRANS();    BootStrapMultiXact();    pfree(buffer);}

说起来 system_identifier 这个东西 ，是随机算出来的一个值：

/*     * Select a hopefully-unique system identifier code for this installation.     * We use the result of gettimeofday(), including the fractional seconds     * field, as being about as unique as we can easily get.  (Think not to     * use random(), since it hasn't been seeded and there's no portable way     * to seed it other than the system clock value...)  The upper half of the     * uint64 value is just the tv_sec part, while the lower half is the XOR     * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness     * unnecessarily if "uint64" is really only 32 bits wide.  A person     * knowing this encoding can determine the initialization time of the     * installation, which could perhaps be useful sometimes.     */    gettimeofday(&tv, NULL);    sysidentifier = ((uint64) tv.tv_sec) << 32;    sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);...    ControlFile->system_identifier = sysidentifier;

 state 在初始化的时候，是有SHUTDOWNED。另外可能的值也都可以在pg_control.h中看到：

/* * System status indicator.  Note this is stored in pg_control; if you change * it, you must bump PG_CONTROL_VERSION */typedef enum DBState{    DB_STARTUP = 0,    DB_SHUTDOWNED,    DB_SHUTDOWNED_IN_RECOVERY,    DB_SHUTDOWNING,    DB_IN_CRASH_RECOVERY,    DB_IN_ARCHIVE_RECOVERY,    DB_IN_PRODUCTION} DBState;

这几个值何时用，状态如何变化，应该是一个很有意思的话题。