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In my previous post I described how the IO resource manager (IORM) in the exadata storage can be used both to guarantee a minimum amount of IO a database can get (which is what is covered in most material available), but also to set a maximum amount of IO. This is what Oracle calls an inter-database resource manager plan. This is set and configured at the cell level using the cellcli with ‘alter iormplan dbplan’.

But Randy Johnson (who is one of the authors of the upcoming book ‘Expert Oracle Exadata’) commented this is not the only way to limit IO: this can be done using the database resource manager (DBRM) too. This means you can set a limit on the CPU resource usage on the database layer, which gets pushed to the storage layer and enforces IO limits according to the limit set on the database layer. Interesting!

Let’s try it!

Here’s a simple query without any resource management:

SQL> select count(*) from cg_var;

COUNT(*)
----------
1596587000

Elapsed: 00:00:44.00

Now create a simple resource manager plan, with one resource group which limits usage to 1%:

BEGIN
DBMS_RESOURCE_MANAGER.CREATE_PENDING_AREA();

DBMS_RESOURCE_MANAGER.CREATE_PLAN(
PLAN => 'TESTPLAN',
COMMENT => 'Testplan! Limit a user to 1 percent');

DBMS_RESOURCE_MANAGER.CREATE_CONSUMER_GROUP (
CONSUMER_GROUP => 'LIMITED',
COMMENT => 'This is a limited group');

DBMS_RESOURCE_MANAGER.CREATE_PLAN_DIRECTIVE(
PLAN => 'TESTPLAN',
GROUP_OR_SUBPLAN => 'LIMITED',
COMMENT => 'This group is limited to 1 percent',
MAX_UTILIZATION_LIMIT => 1);

DBMS_RESOURCE_MANAGER.CREATE_PLAN_DIRECTIVE(
PLAN => 'TESTPLAN',
GROUP_OR_SUBPLAN => 'OTHER_GROUPS',
COMMENT => 'Mandatory group');

DBMS_RESOURCE_MANAGER.SET_CONSUMER_GROUP_MAPPING
(DBMS_RESOURCE_MANAGER.ORACLE_USER, 'CG', 'LIMITED');

DBMS_RESOURCE_MANAGER.VALIDATE_PENDING_AREA();
DBMS_RESOURCE_MANAGER.SUBMIT_PENDING_AREA();
END;
/

Mind the “MAX_UTILIZATION_LIMIT => 1″! Using this plan, all sessions in this database are not limited, except for the Oracle User ‘CG’, which has a maximal utilization limit of 1 percent.

Now grant access to the resource group ‘LIMITED’ for the user ‘CG':

BEGIN
DBMS_RESOURCE_MANAGER_PRIVS.grant_switch_consumer_group(
grantee_name => 'CG',
consumer_group => 'LIMITED',
grant_option => FALSE);
END;
/

And activate the resource manager plan:

ALTER SYSTEM SET RESOURCE_MANAGER_PLAN = 'TESTPLAN' sid='*';

And try the same simple SQL again:

SQL> select count(*) from cg_var;

COUNT(*)
----------
1596587000

Elapsed: 00:36:10.21

This is clearly limited!

The resource management is done on the cell/storage layer. This means the waitevents do not indicate resource management (!). When looking at the waits, the process is mostly waiting for the ‘cell smart table scan’ event.

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