Mssm及assm下索引叶块分裂的测试
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Mssm及assm下索引叶块分裂的测试

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    Mssm及assm下索引叶块分裂的测试 Mssm及assm下索引叶块分裂的测试 Document Transcript

    • 对 Oracle 中索引叶块分裂的测试 本文作者:刘相兵 (liu.maclean@gmail.com)摘要: MSSM 方式是不能避免索引分裂引起的超时问题,Coalesce 合并索引是目前既有的最具可操作性且无副作用的解决方案。在版本 10.2.0.4 未打上相关 one-off 补丁的情况下,分别对 ASSM 和 MSSM 管理模式表空间进行索引分裂测试,经过测试的结论如下:  在 10gr2 版本中 MSSM 方式是不能避免索引分裂引起交易超时问题;  10.2.0.4 上的 one-off 补丁因为目前仅存在 Linux 版本,可以考虑声请 补丁后具体测试(因目前没有补丁所以处于未知状态)。  合并索引是目前最具可行性的解决方案(alter index coalesce)。  最新的 11gr2 中经测试仍存在该问题。具体测试过程如下: 1. 自动段管理模式下的索引块分裂SQL> drop tablespace idx1 including contents and datafiles;Tablespace dropped.SQL> create tablespace idx1 datafile ?/dbs/idx1.dbf size 500M 2 segment space management AUTO 3 extent management local uniform size 10M;--创建自动段管理的表空间Tablespace created.SQL> create table idx1(a number) tablespace idx1;Table created.create index idx1_idx on idx1 (a) tablespace idx1 pctfree 0;Index created. -- 创建实验对象表及索引www.oracledatabase12g.com 第 1 页 11 年 7 月 8 日
    • SQL> insert into idx1 select rownum from all_objects,all_objects where rownum <= 250000; -- 插入 25 万条记录250000 rows created.SQL> commit;Commit complete.SQL>create table idx2 tablespace idx1 as select * from idx1where 1=2;Table created.insert into idx2select * from idx1 where rowid in(select rid from(select rid, rownum rn from(select rowid rid from idx1 where a between 10127 and 243625order by a) --取出后端部分记录,即每 250 条取一条)where mod(rn, 250) = 0)/933 rows created.SQL> commit;Commit complete.SQL> analyze index idx1_idx validate structure; --分析原索引select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 0 -- 未删除情况下 499个叶块SQL> delete from idx1 where a between 10127 and 243625;-- 大量删除commit;233499 rows deleted.www.oracledatabase12g.com 第 2 页 11 年 7 月 8 日
    • SQL> SQL>Commit complete.SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 233499 -- 删除后叶块数量不变SQL> insert into idx1 select * from idx2; --令那些 empty 块,不再 empty,但每个块中只有一到二条记录,空闲率仍为 75-100%commit;933 rows created.Commit complete.SQL> insert into idx1 select 250000+rownum from all_objectswhere rownum <= 126; -- 造成 leaf 块分裂前提SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 997 leaf node splits 997 leaf node 90-10 splits 0 branch node splits 0 queue splits --找出当前会话目前的叶块分裂次数SQL>insert into idx1 values (251000);-- 此处确实叶块分裂1 row created.SQL> commit;www.oracledatabase12g.com 第 3 页 11 年 7 月 8 日
    • Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 998 leaf node splits 998 leaf node 90-10 splits 0 branch node splits 0 queue splits -- 可以看到对比之前的查询多了一个叶块分裂SQL> set linesize 200 pagesize 1500;SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1603 0 271601 271601933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 156 0 82803 82803126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 177 0 3728 37281www.oracledatabase12g.com 第 4 页 11 年 7 月 8 日
    • insert into idx1 values (251000) -- 读了那些实际不空的块,较多buffer_get 1 1409 0 40293 40293933insert into idx1 select * from idx2 1 240842 0 3478341 3478341250000SQL> insert into idx1 values (251001);-- 不分裂的插入1 row created.SQL> commit;Commit complete.SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1603 0 271601 271601933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 156 0 82803 82803126insert into idx1 select 250000+rownum from all_objects wherewww.oracledatabase12g.com 第 5 页 11 年 7 月 8 日
    • rownum <= 126 1 9 0 1640 16401insert into idx1 values (251001) --不分裂的插入,少量 buffer_gets 1 177 0 3728 37281insert into idx1 values (251000) 1 1409 0 40293 40293933insert into idx1 select * from idx2 1 240842 0 3478341 3478341250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 250000 如演示 1 所示,在自动段管理模式下大量删除后插入造成许多块为 75%- 100%空闲率且不完全为空,此后叶块分裂时将引起插入操作的相关前台 进程扫描大量“空块“,若这些块不在内存中(引发物理读)且可能需 要延迟块清除等原因时,减缓了该扫描操作的速度,造成叶块分裂缓慢 , 最终导致了其他 insert 操作被 split 操作所阻塞,出现 enq:tx index contention 等待事件。 2. 手动段管理模式下的索引块分裂SQL> drop tablespace idx1 including contents and datafiles;Tablespace dropped.SQL> create tablespace idx1 datafile ?/dbs/idx1.dbf size 500M 2 segment space management MANUAL-- MSSM 的情况 3 extent management local uniform size 10M;Tablespace created.SQL> create table idx1(a number) tablespace idx1;create index idx1_idx on idx1 (a) tablespace idx1 pctfree 0;Table created.www.oracledatabase12g.com 第 6 页 11 年 7 月 8 日
    • SQL> SQL> insert into idx1 select rownum from all_objects,all_objects where rownum <= 250Index created.SQL> SQL> 000;commit;create table idx2 tablespace idx1 as select * from idx1 where1=2;insert into idx2select * from idx1 where rowid in(select rid from(select rid, rownum rn from(select rowid rid from idx1 where a between 10127 and 243625order by a))where mod(rn, 250) = 0)/commit;250000 rows created.SQL> SQL>Commit complete.SQL> SQL>Table created.SQL> SQL> 2 3 4 5 6 7 8 9933 rows created.SQL> SQL>Commit complete.SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL>www.oracledatabase12g.com 第 7 页 11 年 7 月 8 日
    • BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 0SQL> delete from idx1 where a between 10127 and 243625;233499 rows deleted.SQL> commit;Commit complete.SQL> insert into idx1 select * from idx2;commit;933 rows created.SQL> SQL>Commit complete.SQL> SQL> insert into idx1 select 250000+rownum fromall_objects where rownum <= 126;commit;126 rows created.SQL> SQL>Commit complete.SQL>SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 1496 leaf node splitswww.oracledatabase12g.com 第 8 页 11 年 7 月 8 日
    • 1496 leaf node 90-10 splits 0 branch node splits 0 queue splitsSQL> insert into idx1 values (251000);-- 确实分裂1 row created.SQL> commit;Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 1497 leaf node splits 1497 leaf node 90-10 splits 0 branch node splits 0 queue splits-- 以上与 ASSM 时完全一致SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1553 0 283301 283301933insert into idx2 select * from idx1 where rowid in (select ridwww.oracledatabase12g.com 第 9 页 11 年 7 月 8 日
    • from (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 153 0 78465 78465126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 963 0 10422 104221 -- 比 ASSM 模式下更大量的“空块”读insert into idx1 values (251000) 1 984 0 35615 35615933insert into idx1 select * from idx2 1 238579 0 3468326 3469984250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 250000SQL> insert into idx1 values (251001);1 row created.SQL> commit;Commit complete.SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT------------------------------------------------------------------------------------------------------------------------------www.oracledatabase12g.com 第 10 页 11 年 7 月 8 日
    • -------------------------------------------------------------------------- 1 1553 0 283301 283301933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 153 0 78465 78465126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 7 0 1476 14761insert into idx1 values (251001) --不分裂的情况与 ASSM 时一致 1 963 0 10422 104221insert into idx1 values (251000) 1 984 0 35615 35615933insert into idx1 select * from idx2 1 238579 0 3468326 3469984250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 2500006 rows selected. 如演示 2 所示,MSSM 情况下叶块分裂读取了比 ASSM 模式下更多的“空块 “;MSSM 并不能解决大量删除后叶块分裂需要扫描大量非空块的问题, 实际上可能更糟糕。从理论上讲 MSSM 的 freelist 只能指出那些未达到 pctfree 和曾经到达 pctfree 后来删除记录后使用空间下降到 pctused 的 块(doc:A free list is a list of free data blocks that usually includes blocks existing in a number of different extents within the segment. Free lists are composed of blocks in which free space has not yet reached PCTFREE or used space has shrunkwww.oracledatabase12g.com 第 11 页 11 年 7 月 8 日
    • below PCTUSED.),换而言之 MSSM 模式下”空块“会更多。 3. 自动段管理模式下 coalesce 后的索引块分裂SQL> drop tablespace idx1 including contents and datafiles;Tablespace dropped.SQL> create tablespace idx1 datafile ?/dbs/idx1.dbf size 500M 2 segment space management AUTO-- ASSM 下 coalesce 情况 3 extent management local uniform size 10M;Tablespace created.SQL> create table idx1(a number) tablespace idx1;create index idx1_idx on idx1 (a) tablespace idx1 pctfree 0;Table created.SQL> SQL>Index created.SQL> SQL> insert into idx1 select rownum from all_objects,all_objects where rownum <= 250000;commit;create table idx2 tablespace idx1 as select * from idx1 where1=2;insert into idx2select * from idx1 where rowid in(select rid from(select rid, rownum rn from(select rowid rid from idx1 where a between 10127 and 243625order by a))where mod(rn, 250) = 0)/www.oracledatabase12g.com 第 12 页 11 年 7 月 8 日
    • commit;250000 rows created.SQL> SQL>Commit complete.SQL> SQL>Table created.SQL> SQL> 2 3 4 5 6 7 8 9933 rows created.SQL> SQL>Commit complete.SQL> SQL> SQL>SQL>SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 0SQL> delete from idx1 where a between 10127 and 243625;commit;233499 rows deleted.SQL> SQL>Commit complete.SQL> alter index idx1_idx coalesce;Index altered.www.oracledatabase12g.com 第 13 页 11 年 7 月 8 日
    • SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 33 0-- coalesc 后 lf 块合并了SQL> insert into idx1 select * from idx2;933 rows created.SQL> SQL> commit;Commit complete.SQL>SQL> insert into idx1 select 250000+rownum from all_objectswhere rownum <= 126;commit;126 rows created.SQL> SQL>Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 1999 leaf node splits 1995 leaf node 90-10 splits 0 branch node splits 0 queue splitswww.oracledatabase12g.com 第 14 页 11 年 7 月 8 日
    • SQL> insert into idx1 values (251000);-- 确实分裂1 row created.SQL> commit;Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 2000 leaf node splits 1996 leaf node 90-10 splits 0 branch node splits 0 queue splitsSQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1603 0 268924 268924933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 )www.oracledatabase12g.com 第 15 页 11 年 7 月 8 日
    • 1 156 0 78349 78349126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 23 0 2218 22181 --少量 buffer getsinsert into idx1 values (251000) 1 191 0 15596 15596933insert into idx1 select * from idx2 1 240852 0 3206130 3206130250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 250000SQL> insert into idx1 values (251001);1 row created.SQL> commit;Commit complete.SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1603 0 268924 268924www.oracledatabase12g.com 第 16 页 11 年 7 月 8 日
    • 933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 156 0 78349 78349126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 9 0 1574 15741insert into idx1 values (251001) 1 23 0 2218 22181insert into idx1 values (251000) 1 191 0 15596 15596933insert into idx1 select * from idx2 1 240852 0 3206130 3206130250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 2500006 rows selected. 如演示三所示在删除后进行 coalesce 操作,合并操作将大量空块分离出 了索引结构(move empty out of index structure),之后的叶块分裂仅 读取了少量必要的块。 4. 手动段管理模式下 coalesce 后的索引块分裂SQL> drop tablespace idx1 including contents and datafiles;Tablespace dropped.SQL> create tablespace idx1 datafile ?/dbs/idx1.dbf size 500M 2 segment space management MANUAL-- mssm 情况下 coalesce 3 extent management local uniform size 10M;www.oracledatabase12g.com 第 17 页 11 年 7 月 8 日
    • Tablespace created.SQL> create table idx1(a number) tablespace idx1;create index idx1_idx on idx1 (a) tablespace idx1 pctfree 0;Table created.SQL> SQL> insert into idx1 select rownum from all_objects,all_objects where rownum <= 250Index created.SQL> SQL> 000;commit;create table idx2 tablespace idx1 as select * from idx1 where1=2;insert into idx2select * from idx1 where rowid in(select rid from(select rid, rownum rn from(select rowid rid from idx1 where a between 10127 and 243625order by a))where mod(rn, 250) = 0)/commit;250000 rows created.SQL> SQL>Commit complete.SQL> SQL>Table created.www.oracledatabase12g.com 第 18 页 11 年 7 月 8 日
    • SQL> SQL> 2 3 4 5 6 7 8 9933 rows created.SQL> SQL>Commit complete.SQL> SQL> SQL>SQL>SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 0SQL> delete from idx1 where a between 10127 and 243625;commit;233499 rows deleted.SQL> SQL>Commit complete.SQL> analyze index idx1_idx validate structure;select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 499 233499SQL> alter index idx1_idx coalesce;Index altered.SQL> analyze index idx1_idx validate structure;www.oracledatabase12g.com 第 19 页 11 年 7 月 8 日
    • select blocks,lf_blks,del_lf_rows from index_stats;Index analyzed.SQL> BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 1280 33 0SQL> insert into idx1 select * from idx2;933 rows created.SQL> SQL> commit;Commit complete.SQL>SQL> insert into idx1 select 250000+rownum from all_objectswhere rownum <= 126;commit;126 rows created.SQL> SQL>Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 2502 leaf node splits 2494 leaf node 90-10 splits 0 branch node splits 0 queue splitsSQL> insert into idx1 values (251000); --www.oracledatabase12g.com 第 20 页 11 年 7 月 8 日
    • 确实分裂1 row created.SQL> commit;Commit complete.SQL> select ss.value,sy.name from v$sesstat ss ,v$sysstat sywhere ss.statistic#=sy.statistic# and name like %split% andsid=(select distinct sid from v$mystat); VALUE NAME-------------------------------------------------------------------------- 2503 leaf node splits 2495 leaf node 90-10 splits 0 branch node splits 0 queue splitsSQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1553 0 281059 281059933insert into idx2 select * from idx1 where rowid in (select ridfrom (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 153 0 77817 77817www.oracledatabase12g.com 第 21 页 11 年 7 月 8 日
    • 126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 19 0 2010 20101 -- 少量 buffer getinsert into idx1 values (251000) 1 126 0 15364 15364933insert into idx1 select * from idx2 1 238644 0 3229737 3230569250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 250000SQL> insert into idx1 values (251001);1 row created.SQL> commit;Commit complete.SQL> select executions, buffer_gets, disk_reads, cpu_time,elapsed_time, rows_processed, sql_text from v$sql 2 where sql_text like %insert%idx1% and sql_text not like%v$sql%;EXECUTIONS BUFFER_GETS DISK_READS CPU_TIME ELAPSED_TIMEROWS_PROCESSED---------- ----------- ---------- ---------- --------------------------SQL_TEXT-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 1553 0 281059 281059933insert into idx2 select * from idx1 where rowid in (select ridwww.oracledatabase12g.com 第 22 页 11 年 7 月 8 日
    • from (select rid, rownum rn from (select rowid rid from idx1where a between 10127 and 243625 order by a) ) where mod(rn,250) = 0 ) 1 153 0 77817 77817126insert into idx1 select 250000+rownum from all_objects whererownum <= 126 1 7 0 1460 14601insert into idx1 values (251001) 1 19 0 2010 20101insert into idx1 values (251000) 1 126 0 15364 15364933insert into idx1 select * from idx2 1 238644 0 3229737 3230569250000insert into idx1 select rownum from all_objects, all_objectswhere rownum <= 2500006 rows selected. 如演示 4 所示,MSSM 模式下合并操作与 ASSM 情况下大致一样,合并操作 可以有效解决该问题。 5. Coalesce 合并操作的锁影响SQL> create table coal (t1 int);Table created.SQL> create index pk_t1 on coal(t1);Index created.SQL> begin 2 for i in 1..3000 loop 3 insert into coal values(i); 4 commit;www.oracledatabase12g.com 第 23 页 11 年 7 月 8 日
    • 5 end loop; 6 end; 7 /PL/SQL procedure successfully completed.SQL> delete coal where t1>500;2500 rows deleted.SQL> commit;Commit complete.SQL> analyze index pk_t1 validate structure;Index analyzed. -- 注意 analyze validate 操作会 block 一切 dml 操作SQL> select blocks,lf_blks,del_lf_rows from index_stats; BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 8 6 2500 -- 删除后的状态此时另开一个会话,开始 dml 操作:SQL> update coal set t1=t1+1;500 rows updated.-- 回到原会话SQL> alter index pk_T1 coalesce; -- coalesce 未被阻塞Index altered.-- 在另一个会话中 commit,以便执行 validate structure SQL> analyze index pk_t1 validate structure; Index analyzed. SQL> select blocks,lf_blks,del_lf_rows from index_stats; BLOCKS LF_BLKS DEL_LF_ROWS ---------- ---------- ----------- 8 3 500www.oracledatabase12g.com 第 24 页 11 年 7 月 8 日
    • -- 显然 coalesce 的操作没有涉及有 dml 操作的块在没有 dml 操作的情况下:SQL> truncate table coal;Table truncated.SQL> begin 2 for i in 1..3000 loop 3 insert into coal values(i); 4 commit; 5 end loop; 6 end; 7 /PL/SQL procedure successfully completed.SQL> analyze index pk_t1 validate structure;Index analyzed.SQL> select blocks,lf_blks,del_lf_rows from index_stats; BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 8 6 0SQL> delete coal where t1>500;2500 rows deleted.SQL> commit;Commit complete.SQL> analyze index pk_t1 validate structure;Index analyzed.SQL> select blocks,lf_blks,del_lf_rows from index_stats; BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 8 6 2500www.oracledatabase12g.com 第 25 页 11 年 7 月 8 日
    • SQL> alter index pk_t1 coalesce;Index altered.SQL> analyze index pk_t1 validate structure;Index analyzed.SQL> select blocks,lf_blks,del_lf_rows from index_stats; BLOCKS LF_BLKS DEL_LF_ROWS---------- ---------- ----------- 8 1 0--没有 dml 时,coalesce 操作涉及了所有块 如演示 5 所示 coalesce 会避开 dml 操作涉及的块,但在 coalesec 的短暂 间歇出现在索引上有事务的块不会太多。且 coalesce 操作不会降低索引 高度。 附件是关于 rebuild 及 coalesce 索引操作的详细描述: 6. Coalesce 操作总结 优点:  是 一 种 快 速 的 操 作 , 对 整 体 性 能 影 响 最 小 ( not performance sensitive)。  不会锁表,绕过有事务的索引块。  可以有效解决现有的问题。  不会降低索引高度,引起再次的 root split 缺点:  需要针对个别对象,定期执行合并操作;无法一劳永逸地全局地解 决该问题。 7. Linux 10.2.0.4 上相关补丁的技术交流 Metalink bug 8286901 note 中叙述了一位用户遇到相同的问题并提交了 SR,当时 oracle support 给出了 one-off 补丁,但该用户在 apply 了该补丁后仍未解决问题。 以下为 note 原文:It is similar to bug8286901, but after applied patch8286901, still seeenq txcontentiona with high "failed probes on index block reclamation"Issue encountered by customer and Oracle developer (Stefan Pommerenk).www.oracledatabase12g.com 第 26 页 11 年 7 月 8 日
    • He describes is thus:"Space search performed by the index splitter cant find space in neighboringblocks, and then instead of allocating new space, we go and continue tosearch for space elsewhere, which manifests itself in block reads from disk,block cleanouts, and subsequent blocks written due to aggressive MTTRsetting.""To clarify: the cleanouts are not the problem per se. The culprit seems tobe that the space search performed by the index splitter cant find space inneighboring blocks, and then instead of allocating new space, we go andcontinue to search for space elsewhere, which manifests itself in block readsfrom disk, block cleanouts, and subsequent blocks written due to aggressiveMTTR setting. This action has caused other sessions to get blocked on TXenqueue contention, blocked on the splitting session. Advice was to set 10224www.oracledatabase12g.com 第 27 页 11 年 7 月 8 日
    • trace event for the splitter for a short time only in order to getdiagnostics as to why the space search rejected most blocks.> A secondary symptom are the bitmap level 1 block updates, which may or maynot be related to the space search; Ive not seen them before, maybe becauseI didnt really pay attention :P , but the symptoms seen in the ASH traceindicate its the same problem. Someone in space mgmt has to look at it toconfirm it is the same problem." 与该用户进行了 mail 私下交流,他的回复:I still have a case open with Oracle. I believe that this is abug in the Oracle code. The problem is that it has beendifficult to create a reproducible test case for Oraclesupport. My specific issue was basically put on hold pendingthe results of another customer’s service request thatappeared to have had the same issue, (9034788). Unfortunatelythey couldn’t reproduce the issue in that case either.I believe that there is a correlation between the enq TX –index contention wait event and a spike in the number of‘failed probes on index block reclamation. I have specificallyasked Oracle to explain why there is a spike in the ‘failedprobes on index block reclamation’ during the same time frameas the enq TX index contention wait event, but they have notanswered my question.I was hoping that some investigation by Oracle Support into thefailed probes metric might get someone on the right track todiscovering the bug. That hasn’t happened though.Hi , Thanks for your sharing . The bug (or specific ktspbehave) is fatal in response time sensitive OLTP env. I would like to ask my customer to coalesce those indexwww.oracledatabase12g.com 第 28 页 11 年 7 月 8 日
    • where massive deleted regularly. Thanks for your help again!Yes, I saw that. I have applied patch 8286901 and set the eventfor version 10.2.0.4, but the problem still occursperiodically. And as I mentioned before, we see a correlationbetween enq TX waits and the failed probes on index blockreclamation. Which is why I still think that it is a bug. Iagree that trying to rebuild or coalesce the indexes are simplyattempts to workaround the issue and not solve the root cause.Early on when I started on this issue I did do some index dumpsand could clearly see that we had lots of blocks with only 1 or2 records after our mass delete jobs. I have provided OracleSupport with this information as well as oradump files whilethe problem is occurring, but they don’t seem to be able tofind anything wrong so far.If you are interested in seeing if you are experiencing a high‘failed probes on index block reclamation’ event run thequery below.select SS.snap_id,SS.stat_name,TO_CHAR(S.BEGIN_INTERVAL_TIME, ‘DAY’) DAY,S.BEGIN_INTERVAL_TIME,S.END_INTERVAL_TIME,SS.value,SS.value – LAG(SS.VALUE, 1, ss.value) OVER (ORDER BYSS.SNAP_ID) AS DIFFfrom DBA_HIST_SYSSTAT SS,DBA_HIST_SNAPSHOT Swhere S.SNAP_ID = SS.SNAP_IDAND SS.stat_NAME = ‘failed probes on index block reclamation’ORDER BY SS.SNAP_ID ; 8. 在 11gr2 上的测试www.oracledatabase12g.com 第 29 页 11 年 7 月 8 日
    • --在最新的 11gr2 中进行了测试,仍可以重现该问题(如图单条 insert 引起了 6675 的buffer_gets,这是在更大量数据的情况下)。 我们可以猜测 Oracle 提供的 one-off 补丁中可能是为叶块分裂所会扫描 的“空块”附加了一个上限,在未达到上限的情况下扫描仍会发生。而 在主流的公开的发行版本中 Oracle 不会引入该补丁的内容。尝试在没有 缓存的情况下引起分裂问题,分裂引起了大约 4000 个块的物理读,但该 操作仍在 0.12 秒(有缓存是 0.02 秒,如图)内完成了(该测试使用普 通 ata 硬 盘 , 读 取 速 度 在 100MB/S: Timing buffered disk reads: 306 MB in 3.00 seconds = 101.93 MB/sec);从 1 月 21 日的 ash 视 图 中 可 以 看 到 引 起 split 的 260 会 话 处 于 单 块 读 等 待 (db file sequential read)中,且已等待了 43950us 约等于 44ms;这与良好 io 的经 验值 10ms 左右有较大出入;我们可以确信 io 性能问题也是引发此叶块 分裂延迟如此显性的一个重要因素。具体结论综上所述,MSSM 方式是不能避免索引分裂引起交易超时问题的;而不删除数据的方案在许多对象上不可行; 10.2.0.4 上的 one-off 补丁因为目前仅存在Linux 版本,可以考虑声请补丁后具体测试(因目前没有补丁所以处于未知状态)。Coalesce 合并索引是目前既有的最具可操作性且无副作用的解决方案。www.oracledatabase12g.com 第 30 页 11 年 7 月 8 日