Oracle RAC Top Undo Sessions Test

Every Oracle Database must have a method of maintaining information that is used to roll back, or undo, changes to the database. Such information consists of records of the actions of transactions, primarily before they are committed. These records are collectively referred to as undo.

Undo records are used to:

  • Roll back transactions when a ROLLBACK statement is issued
  • Recover the database
  • Provide read consistency
  • Analyze data as of an earlier point in time by using Oracle Flashback Query
  • Recover from logical corruptions using Oracle Flashback features

When a ROLLBACK statement is issued, undo records are used to undo changes that were made to the database by the uncommitted transaction. During database recovery, undo records are used to undo any uncommitted changes applied from the redo log to the datafiles. Undo records provide read consistency by maintaining the before image of the data for users who are accessing the data at the same time that another user is changing it.

Oracle provides a fully automated mechanism, referred to as automatic undo management, for managing undo information and space. In this management mode, you create an undo tablespace, and the server automatically manages undo segments and space among the various active sessions.

Each instance in the RAC system can only use one undo tablespace at a time. In other words, instances cannot share undo tablespaces. Each instance in the cluster, being an independent transaction-processing environment, maintains its own UNDO area for undo management. The RAC system allows the creation and use of several undo tablespaces.  When the instance is started, it uses the first available undo tablespace. A second instance will use another undo tablespace. Thus, each instance in a RAC system will have exclusive access to a particular undo tablespace at a given time. The undo tablespace cannot be shared among the instances at the same time. Only once an undo tablespace is released by an instance, it can be assigned to another instance. However, all instances can read blocks from any or all undo tablespaces for the purpose of constructing read-consistency images.

You need to closely observe how the sessions to each RAC instance use the undo tablespaces;  this will enable you to proactively detect unusually high/long usage conditions. The RAC Top Undo Sessions test brings such anomalies to light. This test reports the number of sessions (per instance) accessing the undo tablespace and the duration of usage of these sessions, thus indicating excessive usage (if any) of the undo tablespace. The detailed diagnosis capability of the test turns the spotlight on those sessions that are the leading users of the undo tablespace, and provides pointers to the query executed by these sessions. With the help of this information you can identify inefficient queries and fine-tune them, so that potential processing delays and consequent instance slowdowns/crashes can be averted.

Target of the test : Oracle RAC

Agent deploying the test : An internal/remote agent

Outputs of the test : One set of results for each instance of the monitored Oracle RAC.

Configurable parameters for the test
  1. TEST PERIOD - How often should the test be executed.
  2. Host – The host for which the test is to be configured.
  3. Port - The port on which the server is listening.
  4. orasid - The variable name of the oracle instance.
  5. service name - A ServiceName exists for the entire Oracle RAC system. When clients connect to an Oracle cluster using the ServiceName, then the cluster routes the request to any available database instance in the cluster. By default, the service name is set to none. In this case, the test connects to the cluster using the orasid and pulls out the metrics from that database instance which corresponds to that orasid. If a valid service name is specified instead, then, the test will connect to the cluster using that service name, and will be able to pull out metrics from any available database instance in the cluster.

    To know the ServiceName of a cluster, execute the following query on any node in the target cluster:

    select name, value from v$parameter where name =’service_names’

  6. User – In order to monitor an Oracle database server, a special database user account has to be created in every Oracle database instance that requires monitoring. A Click here hyperlink is available in the test configuration page, using which a new oracle database user can be created. Alternatively, you can manually create the special database user. When doing so, ensure that this user is vested with the select_catalog_role and create session privileges.

    The sample script we recommend for user creation (in Oracle database server versions before 12c) for eG monitoring is:

    create user oraeg identified by oraeg ;

    create role oratest;

    grant create session to oratest;

    grant select_catalog_role to oratest;

    grant oratest to oraeg;

    The sample script we recommend for user creation (in Oracle database server 12c) for eG monitoring is:

    alter session set container=<Oracle_service_name>;

    create user <user_name>identified by <user_password> container=current default tablespace <name_of_default_tablespace> temporary tablespace <name_of_temporary_tablespace>;

    Grant create session to <user_name>;                                 

    Grant select_catalog_role to <user_name>;

    The name of this user has to be specified here.

  7. Password – Password of the specified database user
  8. Confirm password – Confirm the password by retyping it here.
  9. ISPASSIVE – If the value chosen is yes, then the Oracle server under consideration is a passive server in an Oracle cluster. No alerts will be generated if the server is not running. Measures will be reported as "Not applicable" by the agent if the server is not up.
  10. To make diagnosis more efficient and accurate, the eG Enterprise embeds an optional detailed diagnostic capability. With this capability, the eG agents can be configured to run detailed, more elaborate tests as and when specific problems are detected. To enable the detailed diagnosis capability of this test for a particular server, choose the On option. To disable the capability, click on the Off option.

    The option to selectively enable/disable the detailed diagnosis capability will be available only if the following conditions are fulfilled:

    • The eG manager license should allow the detailed diagnosis capability
    • Both the normal and abnormal frequencies configured for the detailed diagnosis measures should not be 0.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Average duration:

Indicates the average time taken by the sessions to this instance, to execute queries on the undo tablespace.

Secs

Ideally, the value of this measure should be low. An unusually high value for this measure could indicate that one/more sessions are using the undo tablespace for too long a time. Use the detailed diagnosis of this measure to identify the top sessions in terms of duration of usage of the undo tablespace, and determine the SQL ID of the query executed by each session on that tablespace. Inefficient queries can thus be isolated. Fine-tuning these queries will enable the optimal usage of the undo tablespace.

Query processing will also be delayed if the undo tablespace is improperly sized. If the undo tablespace has insufficient space, many transactions to the tablespace may terminate before completion, and many more transactions may even hang; this will result in a long line of long-running queries.

Also, since Oracle automatically tunes the undo retention period based on undo tablespace size and system activity, if the undo tablespace runs out of space, it will grossly affect the auto retention capability of Oracle, once again causing query failures. When available space for new transactions becomes short, the database begins to overwrite expired undo. If the undo tablespace has no space for new transactions after all expired undo is overwritten, the database may begin overwriting unexpired undo information. If any of this overwritten undo information is required for consistent read in a current long-running query, the query could fail with the snapshot too old error message.

Number of sessions:

Indicates the number of sessions to this instance that are utilizing the undo tablespace.

Number

This serves as a good indicator of the load on the undo tablespace.