PostgreSQL启动过程中的那些事七：初始化共享内存和信号二：shmem中初始化xlog

		
 pg初始化完shmem，给其加上索引"ShmemIndex"后，接着就在shmem里初始化xlog。
1先上个图，看一下函数调用过程梗概，中间略过部分细节
 

 
初始化xlog方法调用流程图
 
2初始化xlog相关结构
        话说main()->…->PostmasterMain()->…->reset_shared() ->CreateSharedMemoryAndSemaphores()>…->XLOGSHmemInit()，初始化控制文件data/global/pg_control相关数据结构及事务日志xlog相关数据结构，相关结构定义在下面。
typedef struct ControlFileData
{
       /*
        * Unique system identifier --- to ensure wematch up xlog files with the
        * installation that produced them.
        */
       uint64           system_identifier;
 
       /*
        * Version identifier information.   Keep these fields at the same offset,
        * especially pg_control_version; they won't bereal useful if they move
        * around.   (Forhistorical reasons they must be 8 bytes into the file
        * rather than immediately at the front.)
        *
        * pg_control_version identifies the format ofpg_control itself.
        * catalog_version_no identifies the format ofthe system catalogs.
        *
        * There are additional version identifiers inindividual files; for
        * example, WAL logs contain per-page magic numbersthat can serve as
        * version cues for the WAL log.
        */
       uint32           pg_control_version;         /* PG_CONTROL_VERSION */
       uint32           catalog_version_no;        /* see catversion.h */
 
       /*
        * System status data
        */
       DBState        state;                   /*see enum above */
       pg_time_t    time;                    /*time stamp of last pg_control update */
       XLogRecPtr  checkPoint;        /*last check point record ptr */
       XLogRecPtr  prevCheckPoint; /* previous check point recordptr */
 
       CheckPoint checkPointCopy; /* copy of last check pointrecord */
 
       /*
        * These two values determine the minimum pointwe must recover up to
        * before starting up:
        *
        * minRecoveryPoint is updated to the latestreplayed LSN whenever we
        * flush a data change during archive recovery.That guards against
        * starting archive recovery, aborting it, andrestarting with an earlier
        * stop location. If we've already flushed datachanges from WAL record X
        * to disk, we mustn't start up until we reachX again. Zero when not
        * doing archive recovery.
        *
        * backupStartPoint is the redo pointer of thebackup start checkpoint, if
        * we are recovering from an online backup andhaven't reached the end of
        * backup yet. It is reset to zero when the endof backup is reached, and
        * we mustn't start up before that. A booleanwould suffice otherwise, but
        * we use the redo pointer as a cross-checkwhen we see an end-of-backup
        * record, to make sure the end-of-backuprecord corresponds the base
        * backup we're recovering from.
        */
       XLogRecPtr  minRecoveryPoint;
       XLogRecPtr  backupStartPoint;
 
       /*
        * Parameter settings that determine if the WALcan be used for archival
        * or hot standby.
        */
       int                 wal_level;
       int                 MaxConnections;
       int                 max_prepared_xacts;
       int                 max_locks_per_xact;
 
       /*
        * This data is used to check for hardware-architecturecompatibility of
        * the database and the backendexecutable.  We need not check endianness
        * explicitly, since the pg_control versionwill surely look wrong to a
        * machine of different endianness, but we doneed to worry about MAXALIGN
        * and floating-point format.  (Note: storage layout nominally also
        * depends on SHORTALIGN and INTALIGN, but inpractice these are the same
        * on all architectures of interest.)
       &nbs

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