這個據說是PostgreSQL的control file。

到底如何呢，先看看改名后如何，把pg_control文件改名，然后啟動 Postgres，運行時得到信息：

[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

對應的源代碼，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\n""Expected to find it in the directory \"%s\",\n""but could not open file \"%s\": %s\n",progname, DataDir, path, strerror(errno));ExitPostmaster(2);}FreeFile(fp);

將 pg_control文件改回原來的名字后，重新啟動PostgreSQL數據庫，沒有問題。

而在main.c中，有如下代碼：

從注釋中可以看到，數據庫中初始化后，會有LC_CTYPE/LC_COLLATE等信息已經寫入到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，有如下代碼：

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

可見，與pg_control文件相對應，在內存中保留著一個內存結構。

它長得是這個樣子：

/** 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;

然后，一個一個地看吧。

總結

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