PostgreSQL学习系列—pg_control之Database|PostgreSQL学习系列—pg_control之Database system identifier PostgreSQL学习系列—pg_control之

[hgps@ps2 bin]$ pg_controldata ../data pg_control version number:942 Catalog version number:201510051 Database system identifier:6548580191788017147 Database cluster state:in production pg_control last modified:Mon 21 May 2018 11:56:42 AM CST Latest checkpoint location:0/2797010 Prior checkpoint location:0/17FAF50 Latest checkpoint's REDO location:0/2797010 Latest checkpoint's REDO WAL file:000000010000000000000002 Latest checkpoint's TimeLineID:1 Latest checkpoint's PrevTimeLineID:1 Latest checkpoint's full_page_writes: on Latest checkpoint's NextXID:0/1836 Latest checkpoint's NextOID:24576 Latest checkpoint's NextMultiXactId:1 Latest checkpoint's NextMultiOffset:0 Latest checkpoint's oldestXID:1825 Latest checkpoint's oldestXID's DB:1 Latest checkpoint's oldestActiveXID:0 Latest checkpoint's oldestMultiXid:1 Latest checkpoint's oldestMulti's DB: 1 Latest checkpoint's oldestCommitTsXid:0 Latest checkpoint's newestCommitTsXid:0 Time of latest checkpoint:Mon 21 May 2018 11:54:48 AM CST Fake LSN counter for unlogged rels:0/1 Minimum recovery ending location:0/0 Min recovery ending loc's timeline:0 Backup start location:0/0 Backup end location:0/0 End-of-backup record required:no wal_level setting:minimal wal_log_hints setting:off max_connections setting:300 max_worker_processes setting:8 max_prepared_xacts setting:0 max_locks_per_xact setting:64 track_commit_timestamp setting:off Maximum data alignment:8 Database block size:8192 Blocks per segment of large relation: 131072 WAL block size:8192 Bytes per WAL segment:16777216 Maximum length of identifiers:64 Maximum columns in an index:32 Maximum size of a TOAST chunk:1996 Size of a large-object chunk:2048 Date/time type storage:64-bit integers Float4 argument passing:by value Float8 argument passing:by value Data page checksum version:0 Data encryption:off

pg_control数据定义如下：

typedef struct ControlFileData { /* * Unique system identifier --- to ensure we match up xlog files with the * installation that produced them. */ uint64system_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. */ uint32pg_control_version; /* PG_CONTROL_VERSION */ uint32catalog_version_no; /* see catversion.h */ /* * System status data */ DBStatestate; /* see enum above */ pg_time_ttime; /* time stamp of last pg_control update */ XLogRecPtrcheckPoint; /* last check point record ptr */ CheckPointcheckPointCopy; /* copy of last check point record */ XLogRecPtrunloggedLSN; /* current fake LSN value, for unlogged rels */ /* * 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. * * backupEndPoint is the backup end location, if we are recovering from an * online backup which was taken from the standby and haven't reached the * end of backup yet. It is initialized to the minimum recovery point in * pg_control which was backed up last. It is reset to zero when the end * of backup is reached, and we mustn't start up before that. * * If backupEndRequired is true, we know for sure that we're restoring * from a backup, and must see a backup-end record before we can safely * start up. If it's false, but backupStartPoint is set, a backup_label * file was found at startup but it may have been a leftover from a stray * pg_start_backup() call, not accompanied by pg_stop_backup(). */ XLogRecPtrminRecoveryPoint; TimeLineIDminRecoveryPointTLI; XLogRecPtrbackupStartPoint; XLogRecPtrbackupEndPoint; boolbackupEndRequired; /* * Parameter settings that determine if the WAL can be used for archival * or hot standby. */ intwal_level; boolwal_log_hints; intMaxConnections; intmax_worker_processes; intmax_prepared_xacts; intmax_locks_per_xact; booltrack_commit_timestamp; /* * 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. */ uint32maxAlign; /* alignment requirement for tuples */ doublefloatFormat; /* constant 1234567.0 */ #define FLOATFORMAT_VALUE1234567.0 /* * This data is used to make sure that configuration of this database is * compatible with the backend executable. */ uint32blcksz; /* data block size for this DB */ uint32relseg_size; /* blocks per segment of large relation */ uint32xlog_blcksz; /* block size within WAL files */ uint32xlog_seg_size; /* size of each WAL segment */ uint32nameDataLen; /* catalog name field width */ uint32indexMaxKeys; /* max number of columns in an index */ uint32toast_max_chunk_size; /* chunk size in TOAST tables */ uint32loblksize; /* chunk size in pg_largeobject */ /* flags indicating pass-by-value status of various types */ boolfloat4ByVal; /* float4 pass-by-value? */ boolfloat8ByVal; /* float8, int8, etc pass-by-value? */ /* Are data pages protected by checksums? Zero if no checksum version */ uint32data_checksum_version; /* * Random nonce, used in authentication requests that need to proceed * based on values that are cluster-unique, like a SASL exchange that * failed at an early stage. */ charmock_authentication_nonce[MOCK_AUTH_NONCE_LEN]; /* CRC of all above ... MUST BE LAST! */ pg_crc32ccrc; } ControlFileData;

pg_control文件首次创建是在src/backend/access/transam/xlog.c中的void BootStrapXLOG(void) 完成。
BootStrapXLOG(void)函数在系统安装时仅执行一次，负责创建pg_control文件以及初始化XLOG文件。
Database system identifier（数据库系统标识符，内部提示为sysid），用于唯一识别Database Cluster，启动、备份或者恢复等过程中会校验pg_control中的Database system identifier与wal文件中的sysid是否相同。
sysid生成的算法为：

uint64 sysidentifier; /* * 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 contains the * tv_usec part (which must fit in 20 bits), plus 12 bits from our current * PID for a little extra uniqueness.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 |= ((uint64) tv.tv_usec) << 12; sysidentifier |= getpid() & 0xFFF;

pg_controldata显示sysid时，基于跨平台展示的考虑，采用char进行了转换：

/* * Format system_identifier and mock_authentication_nonce separately to * keep platform-dependent format code out of the translatable message * string. */ snprintf(sysident_str, sizeof(sysident_str), UINT64_FORMAT, ControlFile->system_identifier);

【PostgreSQL学习系列—pg_control之Database|PostgreSQL学习系列—pg_control之Database system identifier】基于这种机制，可以利用system identifier来推断出数据库群集创建时间：