[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.
*/
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 */
CheckPoint checkPointCopy; /* copy of last check point record */
XLogRecPtr unloggedLSN; /* 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().
*/
XLogRecPtr minRecoveryPoint;
TimeLineID minRecoveryPointTLI;
XLogRecPtr backupStartPoint;
XLogRecPtr backupEndPoint;
bool backupEndRequired;
/*
* Parameter settings that determine if the WAL can be used for archival
* or hot standby.
*/
int wal_level;
bool wal_log_hints;
int MaxConnections;
int max_worker_processes;
int max_prepared_xacts;
int max_locks_per_xact;
bool track_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.
*/
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 */
uint32 loblksize; /* chunk size in pg_largeobject */
/* flags indicating pass-by-value status of various types */
bool float4ByVal; /* float4 pass-by-value? */
bool float8ByVal; /* float8, int8, etc pass-by-value? */
/* Are data pages protected by checksums? Zero if no checksum version */
uint32 data_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.
*/
char mock_authentication_nonce[MOCK_AUTH_NONCE_LEN];
/* CRC of all above ... MUST BE LAST! */
pg_crc32c crc;
} 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);
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：
SELECT to_timestamp(((6548580191788017147>>32)&(2^32 -1)::bigint));
to_timestamp
------------------------
2018-04-26 10:35:41+08
