Archive for November 2011

Tuning Oracle to Make a Query Slower

I had an interesting little project this morning. Of course it takes longer to write it down than to do actually do it, but it was kind of interesting and since I haven’t done a post in quite some time (and it’s the day before Thanksgiving, so it’s pretty quite at the office anyway) I decided to share. One of the Enkitec guys (Tim Fox) was doing a performance comparison between various platforms (Exadata using it’s IB Storage Network, Oracle Database Appliance (ODA) using it’s direct attached storage, and a standard database on a Dell box using EMC fiber channel attached storage). The general test idea was simple – see how the platforms stacked up for a query that required a full scan of a large table. More specifically, what Tim wanted to see was the relative speed at which the various storage platforms could return data. The expectation was that the direct attached storage would be fastest and the fibre channel storage would be slowest (especially since we only had a single 2G HBA). He tested ODA and Exadata and got basically what he expected, but when he went to test the database on the Dell he was surprised that it was actually faster than either of the other two tests. So here’s some output from the initial tests: First the Exadata. It’s an X2 quarter rack with one extra storage server. Note that we had to set cell_offload_processing to false to turn off the Exadata storage optimizations, thus giving us a measurement of the hardware capabilities without the Exadata offloading.

> !sqlp
SQL*Plus: Release Production on Wed Nov 23 11:08:28 2011
Copyright (c) 1982, 2010, Oracle.  All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options
SYS@DEMO1> @uptime
---------------- ----------------- ----------------- ------- ----------
DEMO1            07-NOV-2011 12:37 23-NOV-2011 11:08   15.94    1377058
SYS@DEMO1> set sqlprompt "_USER'@'EXADATA'>' "
SYS@EXADATA> ! cat /etc/redhat-release
Enterprise Linux Enterprise Linux Server release 5.5 (Carthage)
SYS@EXADATA> ! uname -a
Linux 2.6.18- #1 SMP Tue Aug 31 22:41:13 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
SYS@EXADATA> alter session set "_serial_direct_read"=always;
Session altered.
SYS@EXADATA> alter session set cell_offload_processing=false;
Session altered.
SYS@EXADATA> set autotrace on
SYS@EXADATA> set timing on
SYS@EXADATA> select count(*) from instructor.class_sales;
Elapsed: 00:00:43.01
Execution Plan
Plan hash value: 3145879882
| Id  | Operation                  | Name        | Rows  | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT           |             |     1 |   314K  (1)| 00:00:02 |
|   1 |  SORT AGGREGATE            |             |     1 |            |          |
|   2 |   TABLE ACCESS STORAGE FULL| CLASS_SALES |    90M|   314K  (1)| 00:00:02 |
          1  recursive calls
          0  db block gets
    1168567  consistent gets
    1168557  physical reads
          0  redo size
        526  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SYS@EXADATA> set autotrace off
Enter value for sql_text: select count(*) from instructor.class_sales
Enter value for sql_id: 
------------- ------ ---------- ---------- ------------- ------------- ------------- ------------ ----------------------------------------
b2br1x82p9862      0          1          1         43.00          3.16  1,168,557.00    1,168,567 select count(*) from instructor.class_sa
Elapsed: 00:00:00.08

So the test on the Exadata took 43 seconds to read and transport roughly 1 million 8K blocks. The same test on the ODA looked like this: Continue reading ‘Tuning Oracle to Make a Query Slower’ »