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Don Burleson Blog 







Oracle Lock Escalation Tips

Expert Oracle Database Tips by Donald BurlesonMarch 22, 2015

Oracle Lock Management and Escalation

Lock escalation occurs when numerous locks are held at one level of granularity, such as rows, and a database raises the locks to a higher level of granularity, like table. For example, if a single user locks many rows in a table, some databases automatically escalate the user's row locks to a single table. The number of locks is reduced, but the restrictiveness of what is being locked is increased.

Unlike other database products such as IBM DB2 and Microsoft SQL Server, Oracle never escalates locks. Lock escalation greatly increases the likelihood of deadlocks. Imagine the situation where the system is trying to escalate locks on behalf of transaction T1 but cannot because of the locks held by transaction T2. A deadlock is created if transaction T2 also requires lock escalation of the same data before it can proceed.

Now some tips will be provided on how to manage locking issues with clustered Oracle RAC environments.

Lock Management for Oracle RAC

Oracle Real Application Cluster (RAC) environments introduce a special case of how locking activity is managed quite differently than single instance Oracle environments. Since Oracle RAC involves multiple instances that share a common database on shared storage, the gv$ dynamic performance views can be used to monitor the status for locking activities and to resolve lock conflicts within a RAC environment.

Locking Mechanism in Oracle RAC

Database resources within Oracle, such as tables and rows, are represented by enqueues. Enqueues are a special type of lock within Oracle that places a request for a resource into a queue. These can be locked in various modes such as shared or exclusive lock modes. Concurrent locking requests may enter a conflict based upon lock compatibility rules. Enqueue lock resources are accessed externally by querying the dynamic performance view. Lock requests can be viewed by query of the gv$lock dynamic performance view. Next, how locking functions with Oracle RAC environments will be illustrated by a review of common lock conflict types and example.

Locking Conflict Types

Within Oracle environments, lock conflicts are composed of the following two basic types:

  • Local locking conflicts (block level)
  • Global lock conflicts (block level)

Local lock conflicts are limited to conflicting sessions connected to the same instance which may also apply to a single instance within RAC environments. In the v$lock dynamic performance view, the column for BLOCK contains the value of 1 for blocking lock (session).

Global locking conflict (block) occurs when conflicting sessions are connected to different instances for RAC environments only. With RAC environments for Oracle, the v$lock column BLOCK will contain a value of 2 to mark potential conflicts. The value will always appear as 2 for RAC environments unless there is a local conflict present.

The following script displays all sessions that are holding or requesting locking of resources for a particular session. Waiting sessions have a non-zero value for the column GV$LOCK.REQUEST. Resources are identified by the TYPE, ID1, and ID2 columns in the dynamic performance view for gv$lock.

How to Locate the Root Blocker with Lock Problems

The following query provides the code sample along with the query to execute to determine the root blockers causing the lock problems. The solution is to first locate and kill the root blockers. Usually the row with the highest CTIME value, e.g. row L1, will be the starting point for determine the root blocker.

Make sure that there is not another row with the same SID as that found in the L1 column, then kill the root blocker value in L1 column. Another option is to kill the oldest blocking session, which should have the highest CTIME value.



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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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