Troubleshooting hung Oracle databases and sessions with Real-Time ADDM


Oracle® Enterprise Manager (OEM) 12c and 13c includes many performance analysis tools, including a support tool, called OEM Real-Time Automatic Database Diagnostic Monitor (Real-Time ADDM), for the Oracle DBA to use for troubleshooting or tuning real-time, ongoing performance issues. This blog shares knowledge about practical use of Real-Time ADDM to identify and survive an emergency due to any type of database health problems, such as 100% session, process utilization, or exceeding the predefined critical limits setup for input/output (I/O), memory, or interconnect limits. In such cases, Real-Time ADDM is a very handy tool, and provides the capability to do deeper real-time and realistic ADDM analysis of database health, so let’s compare RADDM vs. ADDM.

What is ADDM?

ADDM is a tool provided by Oracle Database (starting with version 10g) to analyze data in the Automatic Workload Repository (AWR) to identify potential performance bottlenecks. For each of the identified issues, it locates the root cause and provides recommendations for correcting the problem.

What is Real-Time ADDM?

In Oracle Enterprise Manager (OEM) Cloud Control 12c, Oracle introduced the Real-Time ADDM tool, which helps to analyze and resolve problems in unresponsive or hung databases that traditionally require you to restart the database. Real-Time ADDM runs through a set of predefined criteria to analyze the current performance of the database. If any of the criteria matches, Real-Time ADDM helps to resolve the identified issues (such as deadlocks, hangs, shared pool contention, and other exception situations) without having to restart the database.

Database administrators (DBAs) can use Real-Time ADDM to analyze database performance automatically, for instance, in a case when you cannot log in to the database because the database is in a hung state and running slowly due to performance issues. In such a situation, Real-Time ADDM identifies the source of SQL contention on global resources.

Real-Time ADDM also performs all realistic database time analysis like ADDM. When you invoke Real-Time ADDM on a database that is experiencing unusually high database activity, it can detect the top performance issues currently affecting the database and find any major consumer of the database time like SQL sessions, application connections, and so on.

The difference between ADDM and Real-Time ADDM

ADDM is provided as the database-level performance diagnosis tool, and Real-Time ADDM is an OEM version 12c plus tool. You need OEM ready with a connection to database to use Real-Time ADDM.

The major difference between ADDM and Real-Time ADDM is that Real-Time ADDM uses a diagnostic-mode connection to access the Active Session History (ASH) data directly in the System Global Area (SGA) of the database instance, bypassing the normal-mode connection and without using any global resources like latches and queues or an excessive number of host resources.

Why use Real-Time ADDM?

The following list provides reasons to use Real-Time ADDM:

  • Analyze current database performance when it is hanging or running slow.
  • Find sources of severe contention.
  • Perform database time analysis to detect top performance issues.
  • Direct the diagnostic connection to SGA.
  • Avoid the need for HANGANALYZE of ORADEBUG (a basic trace utility of Oracle Database) for reading and analyzing large trace files.

What is HANGANALYZE?

HANGANALYZE is an option within ORADEBUG to locate details about hung or blocking sessions. By using the HANGANALYZE option, DBAs can get a connection into a hung database and generate trace files, which they can read or use with trace analysis tools to identify the hung database issues.

Troubleshooting with Real-Time ADDM demonstration

Assuming you have OEM 12c set up with connection to target database, you can use the following steps to employ Real-Time ADDM analysis when your database instance is in a hung state.

Open a terminal session and try to connect to the database. Notice that the session has not yet started. With a connection not established, you can’t log in to the database because of the hung condition as shown in the following image:

Now, go to OEM and under the Target menu, select Databases for Troubleshooting and search for the database name. As shown in the following image, I am searching for CAC****1P.

Click on the database name. It might take a little time, so wait to refresh. In a bit, you see the error message highlighted in the following image:

For this database instance, this is where Real-Time ADDM is going to be very useful for the database administrator (DBA) to diagnose the situation. Let’s start using Real-Time ADDM on that database instance. From the database on page, go to the Performance menu and select Real-Time ADDM, which takes you to the screen shown in the following image. In this situation, Real-Time ADDM uses a direct connection because the instance is hanging.

You must use SYSDBA credentials for the Real-Time ADDM connection in a hung situation, as shown in the following image:

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After the diagnostic mode connection is established, you can read ASH data directly in memory, starting from the previous hour. After login, you can view the top activity at the top of the activity program as shown in the following image:

You can see that the issue which is causing the database to hang. In this example, it relates to the application weight class. To counter this situation, you need to start the diagnostic to get more insights on the issue. Click START to initiate the analysis. Real-Time ADDM looks at the past ten minutes of ASH samples to determine the issues. To stop after 10 minutes, click STOP. After this is done, Real-Time ADDM shows findings based on priority. The following image indicates the analysis under the Findings tab.

In this example, it shows unresolved or session wait chains, causing the database to hang. Recommendations to fix the issue are indicated in the following image:

In this example, Real-Time ADDM recommended (as shown in the preceding image) that you kill a particular session. To get more information about the process that needs to be terminated, go to the Hang Data tab, as shown in the following image:

Click on Final Blockers to see the blocker session summary. As recommended, kill the session for which you have the session details at the database level Session ID (SID), Serial #, and OS Process ID. If you still can’t log in to database, the only option available is to kill the OS Process ID. To do that, open a terminal and kill the session causing issue as shown in the following image:

After that, you can see that the blocked session can proceed in the Real-Time ADDM analysis window. Now try to connect with an SQL Plus connection from terminal, and this time the connection succeeds as shown in the following image:

Conclusion

The Real-Time ADDM tool from OEM Cloud Control 12c is one of Oracle’s best and quickest tools to support critical database health diagnosis. In the case of a hung database, Real-Time ADDM becomes the DBA’s definitive, effective support, especially when compared to the limited usage of ADDM, and ORADEBUG’s HANGANALYZE trace utility. You can apply the previous steps to overcome your database or hung session issues to get your database up and running.

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Tejashkumar Patel

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