那么和xlog相关的还有个查看数据库状态的命令：pg_controldata, 执行结果如下图：
源文件：https://github.com/linwanggm/postxc/blob/master/pg_controldata_modify.c


[wln@localhost data]$ pg_controldata
pg_control version number:            937
Catalog version number:               201306121
Database system identifier:           6087136306838363069
Database cluster state:               in production
pg_control last modified:             2014年11月30日 星期日 10时17分26秒
Latest checkpoint location:           3/1000058
Prior checkpoint location:            3/58
Latest checkpoint's REDO location:    3/1000024
Latest checkpoint's REDO WAL file:    000000010000000300000001
Latest checkpoint's TimeLineID:       1
Latest checkpoint's PrevTimeLineID:   1
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID:          0/1858
Latest checkpoint's NextOID:          16450
Latest checkpoint's NextMultiXactId:  1
Latest checkpoint's NextMultiOffset:  0
Latest checkpoint's oldestXID:        1768
Latest checkpoint's oldestXID's DB:   1
Latest checkpoint's oldestActiveXID:  1858
Latest checkpoint's oldestMultiXid:   1
Latest checkpoint's oldestMulti's DB: 1
Time of latest checkpoint:            2014年11月30日 星期日 10时17分24秒
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
Current wal_level setting:            hot_standby
Current max_connections setting:      100
Current max_prepared_xacts setting:   0
Current max_locks_per_xact setting:   64
Maximum data alignment:               4
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:        2000
Date/time type storage:               64-bit integers
Float4 argument passing:              by value
Float8 argument passing:              by reference
Data page checksum version:           0




这里整理了源码文件（r1.c）：
执行结果：
[wln@localhost data]$ ./r1
-------256
pg_control version number:            937
Database cluster state:               in production
Database cluster state:               in production
Latest checkpoint location:           3/10000EC
Prior checkpoint location:            3/1000058
Latest checkpoint's REDO location:    3/10000B8
Latest checkpoint's REDO WAL file:    000000010000000300000001
Latest checkpoint's TimeLineID:       1
Latest checkpoint's PrevTimeLineID:   1
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID:          0/1858
Latest checkpoint's NextOID:          16450
Latest checkpoint's NextMultiXactId:  1
Latest checkpoint's NextMultiOffset:  0
Latest checkpoint's oldestXID:        1768
Latest checkpoint's oldestXID's DB:   1
Latest checkpoint's oldestActiveXID:  1858
Latest checkpoint's oldestMultiXid:   1
Latest checkpoint's oldestMulti's DB: 1
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
Current wal_level setting:            hot_standby
Current wal_log_hints setting:        on
Current max_connections setting:      0
Current max_worker_processes setting: 64
Current max_prepared_xacts setting:   4
Current max_locks_per_xact setting:   0
Maximum data alignment:               1093850759
Database block size:                  8192
Blocks per segment of large relation: 16777216
WAL block size:                       64
Bytes per WAL segment:                32
Maximum length of identifiers:        2000
Maximum columns in an index:          257
Maximum size of a TOAST chunk:        0
Size of a large-object chunk:         737221047
Date/time type storage:               floating-point numbers
Float4 argument passing:              by reference
Float8 argument passing:              by reference
Data page checksum version:           0


有关xlog的内容请参考文档（postgres xlog）：http://blog.sina.com.cn/s/blog_67d069a90101qv54.html