PostgreSQL数据库状态控制信息——pg_control文件之DBState一

柔光的暖阳◎ 2022-09-01 08:44 241阅读 0赞

## 概述 ##

pg\_control文件是PostgreSQL的control file。到底如何呢，先看看改名后如何，把pg\_control文件改名，然后启动 Postgres，运行时得到信息：

[postgres@pg101 bin]$ postgres: could not find the database system
    Expected to find it in the directory "/usr/local/pgsql/bin/../data",
    but could not open file "/usr/local/pgsql/bin/../data/global/pg_control": No Such file or Directory

对应的源代码，在postmater.c的 checkDataDir方法中：

snprintf(path, sizeof(path), "%s/global/pg_control", DataDir);
            fp = AllocateFile(path, PG_BINARY_R);
            if (fp == NULL) { 
                    write_stderr("%s: could not find the database system\nExpected to find it in the directory \"%s\",\nbut could not open file \"%s\": %s\n", progname, DataDir, path, strerror(errno)); ExitPostmaster(2);
            } 
            FreeFile(fp);

将 pg\_control文件改回原来的名字后，重新启动PostgreSQL数据库，没有问题。  
而在main.c中，有如下代码：  
从注释中可以看到，数据库中初始化后，会有LC\_CTYPE/LC\_COLLATE等信息已经写入到pg\_control文件中。

/* Set up locale information from environment. Note that LC_CTYPE and * LC_COLLATE will be overridden later from pg_control if we are in an * already-initialized database. We set them here so that they will be * available to fill pg_control during initdb. LC_MESSAGES will get set * later during GUC option processing, but we set it here to allow startup * error messages to be localized. */
            set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("postgres"));

在 src/backend/access/transam/xlog.c 中，有如下代码：

/* We maintain an image of pg_control in shared memory. */
    static ControlFileData *ControlFile = NULL;

可见，与pg\_control文件相对应，在内存中保留着一个内存结构。它长得是这个样子：

/* * Contents of pg_control. * * NOTE: try to keep this under 512 bytes so that it will fit on one physical * sector of typical disk drives. This reduces the odds of corruption due to * power failure midway through a write. */
    
    typedef struct ControlFileData
    { 
        /* * Unique system identifier --- to ensure we match 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 be real useful if they move * around. (For historical reasons they must be 8 bytes into the file * rather than immediately at the front.) * * pg_control_version identifies the format of pg_control itself. * catalog_version_no identifies the format of the system catalogs. * * There are additional version identifiers in individual files; for * example, WAL logs contain per-page magic numbers that 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 record ptr */
    
        CheckPoint    checkPointCopy; /* copy of last check point record */
    
        /* * These two values determine the minimum point we must recover up to * before starting up: * * minRecoveryPoint is updated to the latest replayed LSN whenever we * flush a data change during archive recovery. That guards against * starting archive recovery, aborting it, and restarting with an earlier * stop location. If we've already flushed data changes from WAL record X * to disk, we mustn't start up until we reach X again. Zero when not * doing archive recovery. * * backupStartPoint is the redo pointer of the backup start checkpoint, if * we are recovering from an online backup and haven't reached the end of * backup yet. It is reset to zero when the end of backup is reached, and * we mustn't start up before that. A boolean would suffice otherwise, but * we use the redo pointer as a cross-check when we see an end-of-backup * record, to make sure the end-of-backup record corresponds the base * backup we're recovering from. */
        XLogRecPtr    minRecoveryPoint;
        XLogRecPtr    backupStartPoint;
    
        /* * Parameter settings that determine if the WAL can 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-architecture compatibility of * the database and the backend executable. We need not check endianness * explicitly, since the pg_control version will surely look wrong to a * machine of different endianness, but we do need to worry about MAXALIGN * and floating-point format. (Note: storage layout nominally also * depends on SHORTALIGN and INTALIGN, but in practice these are the same * on all architectures of interest.) * * Testing just one double value is not a very bulletproof test for * floating-point compatibility, but it will catch most cases. */
        uint32        maxAlign;        /* alignment requirement for tuples */
        double        floatFormat;    /* constant 1234567.0 */
        #define FLOATFORMAT_VALUE 1234567.0
    
        /* * This data is used to make sure that configuration of this database is * compatible with the backend executable. */
        uint32        blcksz;            /* data block size for this DB */
        uint32        relseg_size;    /* blocks per segment of large relation */
    
        uint32        xlog_blcksz;    /* block size within WAL files */
        uint32        xlog_seg_size;    /* size of each WAL segment */
    
        uint32        nameDataLen;    /* catalog name field width */
        uint32        indexMaxKeys;    /* max number of columns in an index */
    
        uint32        toast_max_chunk_size;    /* chunk size in TOAST tables */
    
        /* flag indicating internal format of timestamp, interval, time */
        bool        enableIntTimes; /* int64 storage enabled? */
    
        /* flags indicating pass-by-value status of various types */
        bool        float4ByVal;    /* float4 pass-by-value? */
        bool        float8ByVal;    /* float8, int8, etc pass-by-value? */
    
        /* CRC of all above ... MUST BE LAST! */
        pg_crc32    crc;
    } ControlFileData;

### 初始化pg\_control ###

/* This func must be called ONCE on system install. It creates pg_control and the initial XLOG segment. */
    void BootStrapXLOG(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 */
        ...
        pfree(buffer);
    }

system\_identifier 这个东西 ，是随机算出来的一个值。 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;

### DB\_IN\_PRODUCTION ###

研究 DBState，先研究 DB\_IN\_PRODUCTION ，看它如何出现，它出现在启动Postmaster时运行的函数处：

/* This must be called ONCE during postmaster or standalone-backend startup */                                            
    void StartupXLOG(void) {                                             
        …                                                                
        /* Read control file and check XLOG status looks valid. Note: in most control paths, *ControlFile is already valid and we need not do ReadControlFile() here, but might as well do it to be sure. */                                        
        ReadControlFile();                                                                                    
        if (ControlFile->state < DB_SHUTDOWNED ||                                        
            ControlFile->state > DB_IN_PRODUCTION ||                                    
            !XRecOffIsValid(ControlFile->checkPoint.xrecoff))                                    
            ereport(FATAL,(errmsg("control file contains invalid data")));                                                                        
        if (ControlFile->state == DB_SHUTDOWNED)                                        
            ereport(LOG, errmsg("database system was shut down at %s", str_time(ControlFile->time))));                    
        else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)                                        
            ereport(LOG,(errmsg("database system was shut down in recovery at %s",                         str_time(ControlFile->time))));                    
        else if (ControlFile->state == DB_SHUTDOWNING)                                        
            ereport(LOG,(errmsg("database system shutdown was interrupted; last known up at %s",                        str_time(ControlFile->time))));                                                                
        else if (ControlFile->state == DB_IN_CRASH_RECOVERY)                                        
            ereport(LOG,(errmsg("database system was interrupted while in recovery at %s", str_time(ControlFile->time)),errhint("This probably means that some data is corrupted and"you will have to use the last backup for recovery.)));                                                                    
        else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)                                        
            ereport(LOG, (errmsg("database system was interrupted while in recovery at log time %s", str_time(ControlFile->checkPointCopy.time)), errhint("If this has occurred more than once some data might be corrupted and you might need to choose an earlier recovery target.")));                                                                      
        else if (ControlFile->state == DB_IN_PRODUCTION)                                        
            ereport(LOG,(errmsg("database system was interrupted; last known up at %s", str_time(ControlFile->time))));                                                                    
        /* This is just to allow attaching to startup process with a debugger */                               
        #ifdef XLOG_REPLAY_DELAY 
        if (ControlFile->state != DB_SHUTDOWNED)      pg_usleep(60000000L);                                    
        #endif 
                                                
        …                                        
        /* Check whether we need to force recovery from WAL. If it appears to have been a clean shutdown and we did not have a recovery.conf file, then assume no recovery needed. */                                        
        if (XLByteLT(checkPoint.redo, RecPtr))                                        
        {                                         
            …                                    
        }                                        
        else if (ControlFile->state != DB_SHUTDOWNED)                                        
            InRecovery = true;                                    
        else if (InArchiveRecovery) {                                         
            /* force recovery due to presence of recovery.conf */                                    
            InRecovery = true;                                    
        }                                        
                                                
        /* REDO */                                        
        if (InRecovery) {                                         
            …                                    
            /* Update pg_control to show that we are recovering and to show the selected checkpoint as the place we are starting from. We also mark pg_control with any minimum recovery stop point obtained from a backup history file. */                                    
            if (InArchiveRecovery)                                    
                ControlFile->state = DB_IN_ARCHIVE_RECOVERY;                                
            else {                                     
                ereport(LOG, (errmsg("database system was not properly shut down; "automatic recovery in progress)));                
                ControlFile->state = DB_IN_CRASH_RECOVERY;                                
            }                                                                                
            …                                    
        }                                                                                    
        …                                                                                 
        /* Okay, we're officially UP. */                                        
        InRecovery = false;                                                                                    
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);                                        
        ControlFile->state = DB_IN_PRODUCTION;                                        
        ControlFile->time = (pg_time_t) time(NULL);                                        
        UpdateControlFile();                                        
        LWLockRelease(ControlFileLock);                                                                                    
        …                                                                                    
    }

可以说，只要是正常启动了，那么就是DB\_IN\_PRODUCTION状态。

https://blog.csdn.net/xichenguan/article/details/109339556