tuning

Comparation between ASM note [ID 373242.1] and note [ID 452924.1]

2011/04/29 by admin 4 Comments

Question:
Oracle Support on the note “Lun Size And Performance Impact With Asm [ID 373242.1]” and on the note “How to Prepare Storage for ASM [ID 452924.1]” say that in other things:

– Maximize the number of disks in a disk group for maximum data distribution and higher I/O bandwidth
– Size alone should not affect the performance of a LUN. The underlying hardware is what counts.
There is no magic number for the LUN size
– For larger LUNs it is recommended using a large ALLOCATION UNIT.

The provieder of the Storage EMC-Vmax say that the LUNS size can be 200 GB for database OLTP.

We believe that the premise or EMC_Vmax contradicts to ORACLE so instead of maximize the disks they pretend minimize the disks
of the DISK GROUP.

You believe that this is to be true for Storage EMC-Vmax.

Answer:
As mentioned in the document 373242.1.LUN size alone should not affect the performance.

Size alone should not affect the performance of a LUN. The underlying hardware is what counts. There is no magic number for the LUN size. Seek the advice of the storage vendor to recommend the best configuration from a raid 1 or 5 perspective for performance and availability since this may vary between vendors. Given a database size and storage hardware you have available, our best practice recommends creating larger LUNs (if possible to reduce LUN management for the sys admins) and create LUNs from separate set of storage arrays (drives) so that LUNs are not sharing drives if possible.

For larger LUNs it is recommended using a large ALLOCATION UNIT.

Filed Under: Oracle Tagged With: ASM, disk, emc, io, tuning

Extract SQL Plan from AWR

2011/04/25 by admin Leave a Comment

之前有用户在AWR中定位到问题SQL语句后想要了解该SQL statement的具体执行计划，于是就用AWR报告中得到的SQL ID去V$SQL等几个动态性能视图中查询，但发现V$SQL或V$SQL_PLAN视图都已经找不到对应SQL ID的记录，一般来说这些语句已经从shared pool共享池中被替换出去了。

这个时候我们可以尝试使用DBMS_XPLAN.DISPLAY_AWR存储过程来将Oracle Automatic Workload Repository自动负载仓库中记录的SQL语句相关信息抽取出来，如:

SQL> set linesize 200 pagesize 2000;

SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------
askmac.cn

SQL> select /* extract_me */ count(*) from maclean;

  COUNT(*)
----------
      9564

SQL> select sql_id from v$sql where sql_text like '%extract_me%' and sql_text not like '%like%';

SQL_ID
-------------
8vff23q8qp9fj

SQL> exec dbms_workload_repository.create_snapshot;
PL/SQL procedure successfully completed.

QL> select * from table(dbms_xplan.display_cursor('8vff23q8qp9fj',0));

PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  8vff23q8qp9fj, child number 0
-------------------------------------
select /* extract_me */ count(*) from maclean

Plan hash value: 1679547536

----------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |       |    34 (100)|          |
|   1 |  SORT AGGREGATE    |         |     1 |            |          |
|   2 |   TABLE ACCESS FULL| MACLEAN | 10438 |    34   (0)| 00:00:01 |
----------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement

18 rows selected.

/* 冲刷共享池后v$SQL/V$SQL_PLAN等动态视图内的记录会消失,
     需要注意的是如果在AWR快照自动生成之前冲刷了共享池，那么可能丢失SQL的执行统计信息
 */

SQL> alter system flush shared_pool;
System altered.

SQL> select * from table(dbms_xplan.display_cursor('8vff23q8qp9fj',0));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------
SQL_ID: 8vff23q8qp9fj, child number: 0 cannot be found

/* 此时就可以利用dbms_xplan.display_awr存储过程来抓取该SQL_ID对应的执行计划 */

SQL>  select * from table(dbms_xplan.display_awr('8vff23q8qp9fj'));

PLAN_TABLE_OUTPUT
--------------------------------------------------
SQL_ID 8vff23q8qp9fj
--------------------
select /* extract_me */ count(*) from maclean

Plan hash value: 1679547536

----------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |       |    34 (100)|          |
|   1 |  SORT AGGREGATE    |         |     1 |            |          |
|   2 |   TABLE ACCESS FULL| MACLEAN | 10438 |    34   (0)| 00:00:01 |
----------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement

18 rows selected.

/* 这里可以代入'all'选项获取更细致的计划信息 */

SQL> select * from table(dbms_xplan.display_awr('8vff23q8qp9fj',null,null,'all'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------
SQL_ID 8vff23q8qp9fj
--------------------
select /* extract_me */ count(*) from maclean

Plan hash value: 1679547536

----------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |       |    34 (100)|          |
|   1 |  SORT AGGREGATE    |         |     1 |            |          |
|   2 |   TABLE ACCESS FULL| MACLEAN | 10438 |    34   (0)| 00:00:01 |
----------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

   1 - SEL$1
   2 - SEL$1 / MACLEAN@SEL$1

Note
-----
   - dynamic sampling used for this statement

24 rows selected.

/* 也可以从 dba_hist_sql_plan/dba_hist_sqltext等awr历史视图中直接观察该SQL ID对应的语句 */

SQL> select OPERATION,OPTIONS,OBJECT_NAME from dba_hist_sql_plan where sql_id='8vff23q8qp9fj';

OPERATION                      OPTIONS                        OBJECT_NAME
------------------------------ ------------------------------ -------------------------------
SELECT STATEMENT
SORT                           AGGREGATE
TABLE ACCESS                   FULL                           MACLEAN

Filed Under: Oracle, Oracle SQL性能调优 Tagged With: AWR, DBMS_XPLAN.DISPLAY_AWR, tuning

Tom Kyte教你如何制造糟糕的Oracle数据库

2011/01/12 by admin 2 Comments

这个文档来的有点迟，不过迟到总比不到要好的多。我们可爱的Tom大叔这次很有爱的教导我们如何制造大量性能、管理具糟糕的，无人愿意管理的Oracle数据库。应当说做到这一点是十分不容易的，你需要从不考虑绑定变量，不考虑遵循任何范式同时往一个表里塞上两三百个列(当然有人真的这样做，而且认为这样做没什么不妥)，你必须不了解Oracle的任何特性，如果你了解你必然对这些特性嗤之以鼻:”我才不会到那些东西，我用自己的代码来完成”，是的你很满足于一次次地重新制造轮子给自己带来的幸福感，等等。想要制造一个彻头彻底、糟糕到无与伦比的Oracle数据库的你还需要加一把劲，这当然也需要文档的指导，Tom这篇就是为这个来的，读罢这篇Worst Practices你或许还不足以制造世界上最糟糕的Oracle数据库，这不要紧！因为中国有大量的开发糟糕Oracle应用(或者其他什么的)的人才，你们可以组成ACOWPG(ALL CHINA Oracle Worst Practices Group)进一步交流这方面的心得。

好吧，以上以上！如果你和我一样是DBA，工作中有一个这样”蓄意”制造麻烦的家伙就已经够呛了，如果有一堆那么就是doomsday了。
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Filed Under: Oracle, Oracle SQL性能调优 Tagged With: Best Practices, tuning