上節介紹了WAL segment file的總體內部結構和其中的部分結構，本節繼續介紹其中的XLOG Record data結構。
一、XLOG Record data
WAL segment file默認大小爲16MB,其內部結構如下圖所示:
注:上一版本的內部結構圖沒有標明prev XLOG Record data,錯誤認爲XLogLongPageHeaderData爲56Btyes,特此更正!
其中XLOG Record data是存儲實際數據的結構,由以下幾部分組成:
1、0..N個XLogRecordBlockHeader,每個XLogRecordBlockHeader對應一個block data;
注意:如設置了BKPBLOCK_HAS_IMAGE標記,則在XLogRecordBlockHeader結構體後跟XLogRecordBlockImageHeader結構體;如設置了BKPIMAGE_HAS_HOLE和 BKPIMAGE_IS_COMPRESSED則在XLogRecordBlockImageHeader後跟XLogRecordBlockCompressHeader結構體;
2、XLogRecordDataHeader[Short|Long]:如數據<256Bytes,則使用Short格式,否則使用Long格式;
3、block data:full-write-block數據,如啓用了壓縮,則壓縮存儲,相關元數據存儲在XLogRecordBlockHeader中的XLogRecordBlockCompressHeader中.
4、main data:(tuple) data/checkpoint等日誌數據.
插入數據時的XLOG Record data內部結構如下圖所示:
下面逐一介紹上述幾個部分,通過使用hexdump工具查看相關數據。
1、XLogRecordBlockHeader
uint8 id
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 80 -n 1
00000050 00 |.|
00000051
塊引用ID爲0x00,即0號Block.
uint8 fork_flags
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 81 -n 1
00000051 20 | |
00000052
值爲0x20,高4位用於標記,即BKPBLOCK_HAS_DATA
uint16 data_length
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 82 -n 2
00000052 1e 00 |..|
00000054
payload bytes = 0x001E,十進制數值爲30.
接下來是RelFileNode
RelFileNode
tablespace/database/relation,均爲Oid類型(unsigned int)
1.tablespace
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 84 -n 4
00000054 7f 06 00 00 |....|
00000058
值爲0x0000067F,十進制值爲1663
表空間爲default
testdb=# select * from pg_tablespace where oid=1663;
spcname | spcowner | spcacl | spcoptions
------------+----------+--------+------------
pg_default | 10 | |
(1 row)
2.database
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 88 -n 4
00000058 12 40 00 00 |.@..|
0000005c
值爲0x00004012,十進制值爲16402,數據庫爲testdb
testdb=# select * from pg_database where oid=16402;
datname | datdba | encoding | datcollate | datctype | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfroze
nxid | datminmxid | dattablespace | datacl
---------+--------+----------+------------+----------+---------------+--------------+--------------+---------------+---------
-----+------------+---------------+--------
testdb | 10 | 6 | C | C | f | t | -1 | 13284 |
561 | 1 | 1663 |
(1 row)
3.relation
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 92 -n 4
0000005c 56 42 00 00 |VB..|
00000060
值爲0x00004256,十進制值爲16982
testdb=# select oid,relfilenode,relname from pg_class where relfilenode = 16982;
oid | relfilenode | relname
-------+-------------+---------
16982 | 16982 | t_jfxx
(1 row)
相應的關係爲t_jfxx
BlockNumber
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 96 -n 4
00000060 85 00 00 00 |....|
00000064
值爲0x00000085,十進制值爲133,這是對應的數據塊號.
2、XLogRecordDataHeaderShort
接下來是XLogRecordDataHeaderShort/Long,由於數據小於256B,使用XLogRecordDataHeaderShort結構
unit8 id
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 100 -n 1
00000064 ff |.|
00000065
值爲0xFF --> XLR_BLOCK_ID_DATA_SHORT 255
uint8 data_length
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 101 -n 1
00000065 03 |.|
00000066
值爲0x03,3個字節,指的是main data的大小,3個字節是xl_heap_insert結構體的大小.
3、block data
XLogRecordDataHeaderShort之後是block data,由兩部分組成:
1.xl_heap_header
2.Tuple data
xl_heap_header
1.uint16 t_infomask2
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 102 -n 2
00000066 03 00 |..|
00000068
t_infomask2值爲0x03,二進制值爲00000000 00000011
2.uint16 t_infomask
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 104 -n 2
00000068 02 08 |..|
0000006a
t_infomask值爲0x0802,二進制值爲00001000 00000010
3.uint8 t_hoff
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 106 -n 1
0000006a 18 |.|
0000006b
t_hoff值(偏移)爲0x18,十進制值爲24
Tuple data
XLOG Record的大小是0x4F,即79B,減去頭部數據XLogRecord(24B) + XLogRecordBlockHeader(20B) + XLogRecordDataHeaderShort(2B) + xl_heap_header(5B) + main data(3B),剩餘25B
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 107 -n 25
0000006b 00 0d 32 30 39 31 39 0f 32 30 31 33 30 37 00 00 |..20919.201307..|
0000007b 00 00 00 00 00 00 03 b3 40 |........@|
00000084
4、main data
這是xl_heap_insert結構體
uint16 OffsetNumber
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 132 -n 2
00000084 26 00 |&.|
00000086
插入的tuple的偏移爲0x0026,十進制爲38
uint8 flags
[xdb@localhost pg_wal]$ hexdump -C 000000010000000100000042 -s 134 -n 1
00000086 00 |.|
00000087
標誌位爲0x00
二、參考資料
WAL Internals Of PostgreSQL
PostgreSQL 源碼解讀（109）- WAL#5（相關數據結構）
PostgreSQL DBA(16) - WAL segment file內部結構
關於結構體佔用空間大小總結