Chapter 8 Workload Exceptions
Tip: Do not set the qualification time too low on skew exceptions. The skew condition should persist long enough to qualify as a problem with the request.
Synchronous and Asynchronous Exception
Checking
TASM does not check for skew synchronously at the end of request steps. Instead, TASM detects skew asynchronously at the end of the Exception Interval period specified in the
Other tab on the General view. TASM does not wait until the end of a request step to detect skew because a long skewed step could overtake system resources.
Exception Actions
When you create an exception, you define what TASM should do automatically if it finds the exception.
Choose from the following actions.
Action
Notification Only
Abort
Abort Selects Only
Change Workload to
Description
Sends notification and takes no other action.
Stops the query.
Stops the query only if it is a SELECT.
Changes the query to the workload you select from the list.
For all exceptions, TASM logs the exception to TDWMExceptionLog and captures the SQL in DBC.DBQLSqlTbl.
Note: A Change Workload to action will be overridden and ignored if the PM/API request
TDWM WD ASSIGNMENT has been issued and it affects this request.
Notifications
You can select one or more of the following Notifications to occur when an exception is detected.
Notification
Send Alert
Run Program
Post to QTable
Description
Specifies the action to trigger.
Specifies the Alerts registered programs to trigger.
The string you enter is posted to the QTable.
Note: To use alerts as a notification option, you must first create the alerts in the Alert Setup portlet.
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Using Alerts or Run Program with Exceptions
Before you can choose the Send Alert or Run Program actions, you must define those actions in the Alert Setup portlet. For more information, see
Using Alerts or Run Program.
Use the Alert Viewer portlet to see the alerts. Alerts include useful details about the request that triggered the exception, such as the host ID, session number, and request number.
Using Post to QTable with Exceptions
The Post to QTable exception action posts a notification to the system notification queue
(DBC.SystemQTbl). For more information, see Using Post to QTable
.
Elapsed Time or Blocked Time Considerations
The Change Workload to: action is not available for the Elapsed Time or Blocked Time exception criteria. The purpose of changing a workload is to put a request where it belongs in the priority hierarchy. Temporary system-wide conditions cause a request to be blocked or run too long. These exceptions do not occur because the request is out of character for the workload.
Elapsed Time or Blocked Time exceptions are better suited to an automated alert. Then the
DBA can investigate the system-wide situation and resolve the problem by identifying the cause of the slowdown, rather than punishing the requests suffering from the situation.
Abort Exception Action Types
Consider the possible implications of an abort before deciding to use that exception action.
The following examples show why an abort could cause issues.
Request Description
A full-table update that has been running for 30 minutes and has only 5 minutes remaining.
Part of a multi-statement request in which the results of one request feed into the next request.
It is considered an exception because of skew.
Effect of an Abort
A rollback would take several times longer to undo than completing the request. Consider instead the Abort on Select option.
Aborting one of the requests could result in inaccurate results.
The wrong request might be aborted due to a false skew detection.
Even if you know that the workload does not receive the type of request specified in the exception, be careful. TASM may abort a request that is important to the business.
Exception Action Conflict Resolution
One request could trigger multiple exceptions. If that happens, TASM performs all exception actions that do not conflict. For example, TASM always executes all Send Alert, Run
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Program, and Post to QTable actions because they do not conflict. TASM always performs the associated logging also.
A conflict occurs when two simultaneous exceptions call for the following actions against a request:
• Abort versus Change Workload
• Change Workload to: Workload A versus Change Workload to: Workload B
TASM follows these rules to resolve exception action conflicts:
• Abort actions take precedence over Change Workload to: actions
• Change Workload to: is applied based on the workload management method the workload uses, in ascending order of precedence:
• Timeshare tier
• SLG tier
• Tactical tier
TASM resolves further exception action conflicts with these rules, which favor the lower workload classification:
Conflict
Two Change Workload to: actions move a request to the Timeshare tier but to workloads on different levels.
Two Change Workload to: actions move a request to the SLG tier but to workloads on different levels in that tier.
TASM Rule
The request moves into the workload on the lower level. If both actions move the request to the same level, the request moves into the workload whose name appears first in the alphabet.
The request moves into the workload on the lower level. If both actions move the request to the same level, the request moves into the workload with the lower workload allocation percentage value. If the two workloads have the same allocation percentage value, the request moves into the workload whose name appears first in the alphabet.
In every case, TASM logs the exception actions that it did not perform into
TdwmExceptionLog and indicates that they were overridden.
Apply Change WD Action Consistently
TASM can enable or disable exceptions for different planned environments. For example, at night TASM might abort a request matching exception criteria, but during the day, TASM might let that request run.
Tip: Teradata recommends that Change Workload to: actions name the same workload across all planned environments. For details, see the Orange Book, Teradata Active System
Management for Linux SLES 11, 541-0009281.
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