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Smart scan on Exadata and direct path reads. 10 July 2011

Posted by David Alejo Marcos in Uncategorized.
4 comments

Couple of days ago I was called to investigate a performance problem on one of our developement databases. People complained of slowness without much indication of what was slow.

Production, DR and UAT are running on a full rack Exadata machine while development runs on a single server.

The problem:

Database is slow. This is what I was given to start with. After monitoring the system I did notice several sessions where performing direct path reads, db file scattered read and some other events.

I am aware of some people having experienced problems with Oracle doing direct path reads instead of db scattered reads so I thought we might be having the same problem.

The solution:

To cut a long story short, direct path reads was not our problem, at least not this time. I managed to find a long running session consuming many of the database resources. The session was running for 7 days, details below:

SQL Text
------------------------------
UPDATE ....
Global Information
------------------------------
 Status                                 :  EXECUTING
 Duration                               :  613427s
Global Stats
======================================================================================
| Elapsed |   Cpu   |    IO    | Concurrency | Buffer | Read | Read  | Write | Write |
| Time(s) | Time(s) | Waits(s) |  Waits(s)   |  Gets  | Reqs | Bytes | Reqs  | Bytes |
======================================================================================
|  617320 |   64631 |   552688 |        0.82 |   619M |  52M |   6TB |   47M |   2TB |
======================================================================================
SQL Plan Monitoring Details (Plan Hash Value=2108859552)
============================================================================================================================================================================================
| Id    |            Operation             |   Name    |  Rows   | Cost  |   Time    | Start  | Execs |   Rows   | Read  | Read  | Write | Write | Mem | Temp | Activity | Activity Detail |
|       |                                  |           | (Estim) |       | Active(s) | Active |       | (Actual) | Reqs  | Bytes | Reqs  | Bytes |     |      |   (%)    |   (# samples)   |
============================================================================================================================================================================================
|     0 | UPDATE STATEMENT                 |           |         |       |           |        |     1 |          |       |       |       |       |     |      |          |                 |
|     1 |   UPDATE                         | TABLE1    |         |       |           |        |     1 |          |       |       |       |       |     |      |          |                 |
|     2 |    FILTER                        |           |         |       |    613362 |    +63 |     1 |        0 |       |       |       |       |     |      |          |                 |
|     3 |     NESTED LOOPS                 |           |         |       |    613362 |    +63 |     1 |     6905 |       |       |       |       |     |      |          |                 |
|     4 |      NESTED LOOPS                |           |       9 | 27146 |    613362 |    +63 |     1 |     108K |       |       |       |       |     |      |          |                 |
|     5 |       SORT UNIQUE                |           |     839 | 24624 |    613405 |    +20 |     1 |     6905 |    38 |   7MB |  1243 | 257MB | 10M | 333M |          |                 |
|     6 |        TABLE ACCESS FULL         | TABLE1    |     839 | 24624 |        44 |    +20 |     1 |       5M |  1029 | 499MB |       |       |     |      |          |                 |
|  -> 7 |       INDEX RANGE SCAN           | INDEX01   |       6 |     2 |    613364 |    +63 |  6905 |     108K |   792 |   6MB |       |       |     |      |          |                 |
|  -> 8 |      TABLE ACCESS BY INDEX ROWID | TABLE1    |       1 |     6 |    613364 |    +63 |  108K |     6905 | 48839 | 382MB |       |       |     |      |          |                 |
|     9 |     FILTER                       |           |         |       |    613334 |    +91 |  6905 |     6904 |       |       |       |       |     |      |          |                 |
|    10 |      HASH GROUP BY               |           |     798 | 24625 |    252603 |    +65 |  6905 |       4G |   24M |   1TB |   47M | 488GB | 14M |   8M |          |                 |
| -> 11 |       TABLE ACCESS FULL          | TABLE1    |     839 | 24624 |    613364 |    +63 |  6905 |       4G |   29M | 416GB |       |       |     |      |          |                 |
============================================================================================================================================================================================

impressive, right?

The query was terminated and it is being reviewed on development but, why didn’t we have problems on prod?

It is my believe the reason is called smart scan on the Exadata storage :D.

Lets have a look how the query looks on Prod:

1.- is it enabled?

PROD> show parameter cell_offload_processing
NAME                     TYPE     VALUE
-----------------------  -------- ------------------------------
cell_offload_processing  boolean  TRUE

2.- Check the explain plan:

SQL>  SELECT * FROM table(DBMS_XPLAN.DISPLAY_CURSOR(sql_id =>'4xxxxxxxj',format =>'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------
| Id  | Operation                      | Name      | E-Rows |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT               |           |        |       |       |          |
|   1 |  UPDATE                        | TABLE1    |        |       |       |          |
|*  2 |   HASH JOIN ANTI               |           |      9 |  3957K|  1689K| 4524K (0)|
|   3 |    NESTED LOOPS                |           |        |       |       |          |
|   4 |     NESTED LOOPS               |           |      9 |       |       |          |
|   5 |      SORT UNIQUE               |           |    839 |  2037K|   607K| 1810K (0)|
|*  6 |       TABLE ACCESS STORAGE FULL| TABLE1    |    839 |       |       |          |
|*  7 |      INDEX RANGE SCAN          | INDEX01   |      6 |       |       |          |
|*  8 |     TABLE ACCESS BY INDEX ROWID| TABLE1    |      1 |       |       |          |
|   9 |    VIEW                        | VW_NSO_1  |    839 |       |       |          |
|  10 |     SORT GROUP BY              |           |    839 |  2250K|   629K| 1999K (0)|
|* 11 |      TABLE ACCESS STORAGE FULL | TABLE1    |    839 |       |       |          |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("COL_ID"=TO_NUMBER("COL_ID"))
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
   6 - storage(("COL01"=:B2 AND NVL("COL02",'N')='N' AND "COL04"=:B3
              AND "COL03"=TO_NUMBER(:B1)))
       filter(("COL01"=:B2 AND NVL("COL02",'N')='N' AND "COL04"=:B3
              AND "COL03"=TO_NUMBER(:B1)))
   7 - access("COL05"="COL05" AND "COL06"="COL06" AND
              "COL07"="COL07")
   8 - filter((NVL("COL02",'N')='N' AND "COL08"="COL08"))
  11 - storage(("COL01"=:B2 AND NVL("COL02",'N')='N' AND "COL04"=:B3
              AND "COL03"=TO_NUMBER(:B1)))
       filter(("COL01"=:B2 AND NVL("COL02",'N')='N' AND "COL04"=:B3
              AND "COL03"=TO_NUMBER(:B1)))

On the plan_table_output, we see storage, indicating smart scan was being used, being the runtime 73 seconds:

PROD> select sql_id, executions, elapsed_time/1000000 from gv$sql
  2  where sql_id = '4xxxxxxxj';
SQL_ID        EXECUTIONS ELAPSED_TIME/1000000
------------- ---------- --------------------
'4xxxxxxxj'          1            73.166979

I must say, I am quite impressed with Exadata but, would it make it more difficult to spot bad queries?

As always, comments are welcome.

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Oracle DBAs and Bon Jovi 23 July 2010

Posted by David Alejo Marcos in Uncategorized.
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Hard to believe, but Bon Jovi is part of an Oracle DBA life…

This conversation happened during the CAB meeting where we review and approve changes for the weekend. Bon Jovi’s songs in bold:

You always work until late these days and always on weekends;
I know It’s my life and I do not think I will be finishing early. At this rate I ‘ll sleep when I am dead.
Keep the faith.
I think it will be a miracle if all works as expected, unless I misunderstood something; pretty sure we will be livin’ on a prayer.
I believe in you, but I’ll be there for you if you need any help.
ok, have a nice day.

As always, comments are welcome.

Today is Friday 30 October 2009

Posted by David Alejo Marcos in Uncategorized.
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And I decided to start a blog.

The reason is quite simple. A good friend and colleague of mine has suggested to share several, in our opinion, interesting test we have been doing with our systems recently.

In short, and in no specific order:

1.- Backups take longer than expected.

2.- Migrate 1.5 TB10.2.0.3 Oracle RAC 2 nodes on Linux RedHat 32b to 10.2.0.4 RAC 3 nodes on Linux RedHat 64b including physical standby database in the US.

3.- interesting query to transform:

from

row1: value1, value2, value3

row2:value4, value5

to

value1

value2

value3

value4

value5