概述
近期一个客户的一张单表，每天delete7天前的数据，每天的数据增量没什么变化，理论上来说，delete释放的空间是可重用的，但发现该表段最近一直在增长，现在大小为300G，170G的75% – 100% free space blocks.通常通过 APPEND，直接路径加载的方式会直接扩展SEGMENT，不会重用DELETE的空间，但客户的系统排除了APPEND INSERT。
测试目的
我们知道ASSM的表INSERT查找可用空间机制，会在L2 Hint for inserts指上的L2块，再通过算法确定L1块，查找存在可用空间的数据块，完成INSERT。
客户的系统里，发现一个现象，DUMP SEGMENT HEADER，发现L2 Hint for inserts一直指向最后一个L2块。
代码语言：javascript复制 Segment Type: 1 nl2: 37 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x1fd54208 <<<<
Last Level 1 BMB: 0x1fd78207
Last Level II BMB: 0x1fd54208 <<<<
Last Level III BMB: 0x00000000
Second Level Bitmap block DBAs
--------------------------------------------------------
DBA 1: 0x15c00099
省略
DBA 36: 0x1fc82208
DBA 37: 0x1fd54208 <<<<如果ORACLE只是通过L2块查找可用空间，L2 Hint for inserts总是指向最后的L2的话，之前L2块所管理的数据块上数据被DELETE掉，那也没有重用的机会。正常的系统不应该是这样的，下面在我的机器上做一个测试，验证存在多个L2块的系统上，如果前面的L2块管理的块上数据DELETE掉，在表扩展之前，会修改 L2 Hint for inserts的指向，到前面有可用空间的L2，从而重用DELETE释放的空间。
测试步骤如下： 1、创建测试表
代码语言：javascript复制SQL> create table TEST.tab1(id number,c varchar2(1000)) tablespace users;
Table created.2、INSERT数据，直接产生1个以上L2块为止。我这里有两个L2
代码语言：javascript复制insert into TEST.tab1 select 0*100000 rownum,rpad('a',1000,'a') from dual connect by rownum<100000;
insert into TEST.tab1 select 1*100000 rownum,rpad('a',1000,'a') from dual connect by rownum<100000;
insert into TEST.tab1 select 40*100000 rownum,rpad('a',1000,'a') from dual connect by rownum<100000;
这里INSERT了大400万数据
SQL> @seg test.tab1
SEG_MB OWNER SEGMENT_NAME SEG_PART_NAME SEGMENT_TYPE SEG_TABLESPACE_NAME BLOCKS HDRFIL HDRBLK
---------- -------------------- ------------------------------ ------------------------------ -------------------- ------------------------------ ---------- ---------- ----------
4608 TEST TAB1 TABLE USERS 589824 23 130
SQL>
SQL> alter system dump datafile 23 block 130;
System altered.
Segment Type: 1 nl2: 2 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x05c82088 <<<<
Last Level 1 BMB: 0x05c8e087
Last Level II BMB: 0x05c82088 <<<<
Last Level III BMB: 0x00000000
Map Header:: next 0x00000000 #extents: 255 obj#: 73129 flag: 0x10000000
Second Level Bitmap block DBAs
--------------------------------------------------------
DBA 1: 0x05c00081
DBA 2: 0x05c82088 <<<<3、当前的空间实用情况
代码语言：javascript复制set serverout on
exec show_space('TAB1','TEST');
Total Blocks............................589824
Total Bytes.............................4831838208
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................23
Last Used Ext BlockId...................581760
Last Used Block.........................8192
*************************************************
The segment is analyzed
0% -- 25% free space blocks.............0
0% -- 25% free space bytes..............0
25% -- 50% free space blocks............1
25% -- 50% free space bytes.............8192
50% -- 75% free space blocks............0
50% -- 75% free space bytes.............0
75% -- 100% free space blocks...........237
75% -- 100% free space bytes............1941504
Unused Blocks...........................2816
Unused Bytes............................23068672
Total Blocks............................585709
Total bytes.............................4798128128
PL/SQL procedure successfully completed.4、 DELETE 50万数据，因为我INSERT是顺序的，DELETE id<500000的数据，应该释放的是段的前面的一批BLOCKS
代码语言：javascript复制SQL> delete TEST.tab1 where id<500000;
499995 rows deleted.
commit;5、再查看空间使用情况 75% – 100% free space blocks从DELETE前的237 增长到 71665。
代码语言：javascript复制set serverout on
exec show_space('TAB1','TEST');
SQL>
SQL> Total Blocks............................589824
Total Bytes.............................4831838208
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................23
Last Used Ext BlockId...................581760
Last Used Block.........................8192
*************************************************
The segment is analyzed
0% -- 25% free space blocks.............0
0% -- 25% free space bytes..............0
25% -- 50% free space blocks............1
25% -- 50% free space bytes.............8192
50% -- 75% free space blocks............0
50% -- 75% free space bytes.............0
75% -- 100% free space blocks...........71665 <<<<
75% -- 100% free space bytes............587079680
Unused Blocks...........................2816
Unused Bytes............................23068672
Total Blocks............................514281
Total bytes.............................4212989952
PL/SQL procedure successfully completed.
SQL>6、INSERT 999行数据进行测试 因为当前 L2 Hint for inserts指向的L2上仍然有可用空间，会发现下面的INSERT并没有使用DELETE释放的这部分空间。
代码语言：javascript复制SQL> insert into TEST.tab1 select 0 rownum,rpad('a',1000,'a') from dual connect by rownum<1000;
999 rows created.
SQL> SQL> commit;
Commit complete.
SQL> select EXTENT_ID,FILE_ID,BLOCK_ID,BLOCKS from dba_extents where tablespace_name='USERS' and owner='TEST' and segment_name='TAB1' order by extent_id;
EXTENT_ID FILE_ID BLOCK_ID BLOCKS
---------- ---------- ---------- ----------
0 23 128 8
1 23 136 8
2 23 144 8
3 23 152 8
4 23 160 8
5 23 168 8
6 23 176 8
7 23 184 8
8 23 192 8
9 23 200 8
10 23 208 8
11 23 216 8
12 23 224 8
13 23 232 8
14 23 240 8
15 23 248 8
16 23 256 128
17 23 384 128
18 23 512 128
19 23 640 128
20 23 768 128
21 23 896 128
22 23 1024 128
23 23 1152 128
....
----该段的前部分块上并没有我们刚刚INSERT的数据，说明没有重用DELETE的空间。
SQL> select id,dbms_rowid.rowid_relative_fno(rowid) file_id,dbms_rowid.rowid_block_number(rowid) block_id from TEST.tab1 where
2 rowid >=dbms_rowid.rowid_create(1,73129,23,127,1) and rowid <=dbms_rowid.rowid_create(1,73129,23,1024,500)
3 and id<100;
no rows selected7、继续INSERT 10万行数据 因为最后的L2块上，高水位下面没有这么多的空间存放这10万行数据，可以看到这次重用了之前DELETE释放的空间，同时看到L2 Hint for inserts现在指向到了第一个L2块。
代码语言：javascript复制SQL> insert into TEST.tab1 select 0 rownum,rpad('a',1000,'a') from dual connect by rownum<100000;
99999 rows created.
SQL> commit;
Commit complete.
----INSERT 10万行后，Total Blocks 589824，表总BLOCKS没有增长，75% -- 100% free space blocks从71665减少到60053
SQL> set serverout on
SQL> exec show_space('TAB1','TEST');
Total Blocks............................589824
Total Bytes.............................4831838208
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................23
Last Used Ext BlockId...................581760
Last Used Block.........................8192
*************************************************
The segment is analyzed
0% -- 25% free space blocks.............1
0% -- 25% free space bytes..............8192
25% -- 50% free space blocks............0
25% -- 50% free space bytes.............0
50% -- 75% free space blocks............0
50% -- 75% free space bytes.............0
75% -- 100% free space blocks...........60053
75% -- 100% free space bytes............491954176
Unused Blocks...........................0
Unused Bytes............................0
Total Blocks............................528709
Total bytes.............................4331184128
PL/SQL procedure successfully completed.
---可以看到前段的1000个块上有我们刚刚INSERT的数据，说明重用了DELETE释放的空间
SQL> select id,dbms_rowid.rowid_relative_fno(rowid) file_id,dbms_rowid.rowid_block_number(rowid) block_id from TEST.tab1 where
2 rowid >=dbms_rowid.rowid_create(1,73129,23,127,1) and rowid <=dbms_rowid.rowid_create(1,73129,23,10240,500)
3 and id<100000 and rownum<100;
ID FILE_ID BLOCK_ID
---------- ---------- ----------
20445 23 177
20446 23 177
20447 23 177
20448 23 177
20449 23 177
20450 23 177
20451 23 177
。。。。。。。。
20418 23 188
20419 23 188
20420 23 188
20421 23 188
20422 23 188
20423 23 188
20424 23 189
20425 23 189
20426 23 189
20427 23 189
20428 23 189
20429 23 189
20430 23 189
20431 23 190
20432 23 190
20433 23 190
20434 23 190
20435 23 190
20436 23 190
20437 23 190
20438 23 191
99 rows selected.
SQL>
SQL> alter system dump datafile 23 block 130;
System altered.
L2 Hint for inserts的指向也从第二个L2块，换成了第一个L2块。
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 255 #blocks: 589824
last map 0x00000000 #maps: 0 offset: 2716
Highwater:: 0x05c90080 ext#: 254 blk#: 8192 ext size: 8192
#blocks in seg. hdr's freelists: 0
#blocks below: 588763
mapblk 0x00000000 offset: 254
Unlocked
--------------------------------------------------------
Low HighWater Mark :
Highwater:: 0x05c90080 ext#: 254 blk#: 8192 ext size: 8192
#blocks in seg. hdr's freelists: 0
#blocks below: 588763
mapblk 0x00000000 offset: 254
Level 1 BMB for High HWM block: 0x05c8e087
Level 1 BMB for Low HWM block: 0x05c8e087
--------------------------------------------------------
Segment Type: 1 nl2: 2 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x05c00081 <<<<<<
Last Level 1 BMB: 0x05c8e087
Last Level II BMB: 0x05c82088
Last Level III BMB: 0x00000000
Map Header:: next 0x00000000 #extents: 255 obj#: 73129 flag: 0x10000000
Inc # 0
Second Level Bitmap block DBAs
--------------------------------------------------------
DBA 1: 0x05c00081 <<<<<
DBA 2: 0x05c82088
总结
猜测正常情况下，L2 Hint for inserts指向的L2块找不到可用空间的时侯，在尝试ALLOCATE新的EXTENT的前，会去判断之前的L2块有没有可用空间。如果有，会修改L2 Hint for inserts，去使用之前的L2管理的块上释放出来的空间。 客户环境中，大量75% – 100% free space blocks的情况下，INSERT不重用释放的空间问题，还需要继续分析！
作者
范计杰，云和恩墨技术顾问，5年大型ORACLE数据库维护经验，擅长性能调优、故障处理等。
墨天轮原文链接：https://www.modb.pro/db/43425（复制到浏览器或者点击“阅读原文”立即查看）
