一、概念介绍

Oracle ASSM(Automatic Segment Space Managed)使用位图来管理数据库中的空间，具有管理简单、高并发等优点，ASSM的整体结构是3层位图块+数据块，即4层的树状结构，如下所示，

Oracle ASSM 中Segment Header内部结构由Cache Header、Header Control及L2 BMB array三层结构组成，Oracle数据块类型代码为“type: 0x23=PAGETABLE SEGMENT HEADER”,由于bbed工具不支持查询Segment Header的物理结构，本节主要使用C语言解析Segment Header物理结构。

二、使用bbed查看PAGETABLE SEGMENT HEADER结构

SQL> create table hsql.orastar_1(c_char char(10),c_vchar varchar2(10));

Table created.

SQL> insert into hsql.orastar_1 values('a','a');

1 row created.

SQL> insert into hsql.orastar_1 values('b','bb');

1 row created.

SQL> commit;

Commit complete.

set linesize 200 pagesize 200

col owner for a10

col segment_name for a10

select owner,segment_name,header_file,header_block,SEGMENT_TYPE from dba_segments where segment_name='ORASTAR_1';

OWNER SEGMENT_NA HEADER_FILE HEADER_BLOCK SEGMENT_TYPE

---------- ---------- ----------- ------------ ------------------

HSQL ORASTAR_1 5 1698 TABLE

BBED: Release 2.0.0.0.0 - Limited Production on Fri Jan 10 18:45:52 2020

************* !!! For Oracle Internal Use only !!! ***************

BBED> set dba 5,1698

DBA 0x014006a2 (20973218 5,1698)

BBED> map /v

File: /oradata/epmsn/hsql01.dbf (5)

Block: 1698 Dba:0x014006a2

------------------------------------------------------------

BBED-00400: invalid blocktype (35)

BBED>

三、dump结构解析

alter session set tracefile_identifier='orastar_01';

oradebug setmypid

alter system dump datafile 5 block 1698;

oradebug close_trace

oradebug tracefile_name

四、二进制结构解析

五、工具解析

使用C语言结构体解析该数据块结构，解析后的物理结构如下所示，

结构	解释
struct kcbh, 20 bytes @0
ub1 type_kcbh @0 0x23	Block type
ub1 frmt_kcbh @1 0xa2	Block format a2=oracle 112
ub1 spare1_kcbh @2 0x00	not used
ub1 spare2_kcbh @3 0x00	not used
ub4 rdba_kcbh @4 0x01400a0a	RDBA-Relative Data Block Address
ub4 bas_kcbh @8 0x0029cbc7	SCN Base
ub2 wrp_kcbh @12 0x0000	SCN Wrap
ub1 seq_kcbh @14 0x02	sequence number
ub1 flg_kcbh @15 0x04	Flag: 0x01 New Block 0x02 Delayed Loging Change advanced SCN/seq 0x04 Check value saved-block XOR's to zero 0x08 Temporary block
ub2 chkval_kcbh @16 0xe395	Optional block checksum(if DB_BLOCK_CHECKSUM=TRUE)
ub2 spare3_kcbh @18 0x0000	not used
struct ech(Extent Control Header) @36	Extent Control Header
ub4 extents_ech @36 0x00000012	#extents
ub4 blocks_ech @40 0x00000180	#blocks
ub4 offset_ech @44 0x00000a9c	offset
ub4 blk_ech @52 0x00000080	blk#
ub4 extsize_ech @56 0x00000080	ext size
ub4 hw_ech @60 0x01401f80	Highwater
ub4 lhwmext_ech @92 0x00000010	ext# of Low HighWater Mark
ub4 lhwmblk_ech @96 0x00000010	blk# of Low HighWater Mark
ub4 lhwmextsize_ech @100 0x00000080	ext size of Low HighWater Mark
ub4 lhwm_ech @104 0x01401e90	Low HighWater Mark
ub4 lhwmbmb1_ech @124 0x01401e80	Level 1 BMB for Low HWM block
ub4 hhwmbmb1_ech @128 0x01401f01	Level 1 BMB for High HWM block
ub4 segtype_ech @208 0x00000001	Segment Type
ub4 blksz_ech @212 0x00002000	blksz
ub4 fbsz_echo @216 0x00000000	fbsz
ub4 l2asoffset_ech @220 0x00001434	L2 Array start offset
ub4 firstbmb3_echo @224 0x00000000	First Level 3 BMB
ub4 l2hfinsert_ech @228 0x01400a09	L2 Hint for inserts
ub4 nl2_echo @232 0x00000001	nl2
ub4 lastBMB1_echo @236 0x01401f01	Last Level 1 BMB
ub4 lastBMB2_echo @240 0x01400a09	Last Level 2 BMB
ub4 lastBMB3_echo @244 0x00000000	Last Level 3 BMB
ub4 mhnext_echo @260 0x00000000	next of Map Header
ub4 mhext_echo @264 0x00000012	#extents of Map Header
ub4 mhobj_ech @272 0x00015781	obj# of Map Header
ub4 mhflag_ech @276 0x10000000	flag of Map Header
struct em(Extent Map) @280	Extent Map
ub4 dba_em[0] @280 0x01400a08 ub4 len_em[0] @284 0x00000008
ub4 dba_em[1] @288 0x01400a10 ub4 len_em[1] @292 0x00000008
ub4 dba_em[2] @296 0x01400a18 ub4 len_em[2] @300 0x00000008
ub4 dba_em[3] @304 0x01400a20 ub4 len_em[3] @308 0x00000008
ub4 dba_em[4] @312 0x01400a28 ub4 len_em[4] @316 0x00000008
ub4 dba_em[5] @320 0x01400a30 ub4 len_em[5] @324 0x00000008
ub4 dba_em[6] @328 0x01400a38 ub4 len_em[6] @332 0x00000008
ub4 dba_em[7] @336 0x01400a40 ub4 len_em[7] @340 0x00000008
ub4 dba_em[8] @344 0x01400a48 ub4 len_em[8] @348 0x00000008
ub4 dba_em[9] @352 0x01400a50 ub4 len_em[9] @356 0x00000008
ub4 dba_em[10] @360 0x01400a58 ub4 len_em[10] @364 0x00000008
ub4 dba_em[11] @368 0x01400a60 ub4 len_em[11] @372 0x00000008
ub4 dba_em[12] @376 0x01400a68 ub4 len_em[12] @380 0x00000008
ub4 dba_em[13] @384 0x01400a70 ub4 len_em[13] @388 0x00000008
ub4 dba_em[14] @392 0x01400a78 ub4 len_em[14] @396 0x00000008
ub4 dba_em[15] @400 0x01401e00 ub4 len_em[15] @404 0x00000008
ub4 dba_em[16] @408 0x01401e80 ub4 len_em[16] @412 0x00000080
ub4 dba_em[17] @416 0x01401f00 ub4 len_em[17] @420 0x00000080
struct am(Auxillary Map) @2736	Auxillary Map
ub4 fdba_am[0] @2736 0x01400a08 ub4 datadba_am[0] @2740 0x01400a0b
ub4 fdba_am[1] @2744 0x01400a08 ub4 datadba_am[1] @2748 0x01400a10
ub4 fdba_am[2] @2752 0x01400a18 ub4 datadba_am[2] @2756 0x01400a19
ub4 fdba_am[3] @2760 0x01400a18 ub4 datadba_am[3] @2764 0x01400a20
ub4 fdba_am[4] @2768 0x01400a28 ub4 datadba_am[4] @2772 0x01400a29
ub4 fdba_am[5] @2776 0x01400a28 ub4 datadba_am[5] @2780 0x01400a30
ub4 fdba_am[6] @2784 0x01400a38 ub4 datadba_am[6] @2788 0x01400a39
ub4 fdba_am[7] @2792 0x01400a38 ub4 datadba_am[7] @2796 0x01400a40
ub4 fdba_am[8] @2800 0x01400a48 ub4 datadba_am[8] @2804 0x01400a49
ub4 fdba_am[9] @2808 0x01400a48 ub4 datadba_am[9] @2812 0x01400a50
ub4 fdba_am[10] @2816 0x01400a58 ub4 datadba_am[10] @2820 0x01400a59
ub4 fdba_am[11] @2824 0x01400a58 ub4 datadba_am[11] @2828 0x01400a60
ub4 fdba_am[12] @2832 0x01400a68 ub4 datadba_am[12] @2836 0x01400a69
ub4 fdba_am[13] @2840 0x01400a68 ub4 datadba_am[13] @2844 0x01400a70
ub4 fdba_am[14] @2848 0x01400a78 ub4 datadba_am[14] @2852 0x01400a79
ub4 fdba_am[15] @2856 0x01400a78 ub4 datadba_am[15] @2860 0x01401e00
ub4 fdba_am[16] @2864 0x01401e80 ub4 datadba_am[16] @2868 0x01401e82
ub4 fdba_am[17] @2872 0x01401f00 ub4 datadba_am[17] @2876 0x01401f02
ub4 slbbdba1_am @5192 0x01400a09	Second Level Bitmap block DBAs

六、说明

1、以上内容为个人多次测试结果，由于个人原因，如有分析不足之处还请见谅及指正。

2、文章涉及内容，请勿在生产环境模拟。

勤学如春起之苗，不见其增，日有所长；辍学如磨刀之石，不见其损，日有所亏。 --陶渊明

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