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Don Burleson Blog 







Removing Oracle RAC sequence contention 

Oracle Database Tips by Donald BurlesonFebruary 14, 2015

Question:  I am deploying an application on a 5 node RAC cluster and the application will generate sequences on each instance.  What are the most common ways to reduce contention on busy RAC database where there is contention for sequence generation?

Answer:  Sequences in Oracle are the same as a single instance, and the RAC internals will manage the uniqueness of the sequence.  First, see these Oracle best practices for Oracle RAC sequence usage.  

However, it's important to understand that Sequence numbers may be out of order when many nodes are simultaneously generating sequences.  Conversely, if only one node's application is generating the sequence numbers, then we will not see this contention.

The "contention that we see on busy RAC systems (with high insert DML are because each application of each node is competing for resources that exist on the database.  Fortunately, Oracle has several tools for relieving Oracle RAC sequence contention:

  • Use the sequence "cache" option: Oracle has a method for  caching frequently reference sequences, and you can also cache sequences with n-way Streams replication for fast access to sequence values.  Caching sequences is especially important for high-DML applications with lots on insert and update activity.  You can easily cache as sequence with the "add/alter sequence xxx cache" command.  The "cache" clause caches the specified number of sequence values into the buffers in the SGA, speeding-up sequence access speed.

  •  Use the noorder sequence clause:  When creating sequences for a RAC environment, DBAs should use the noorder keyword to avoid an additional cause of SQ enqueue contention that is forced ordering of queued sequence values.  In RAC, a best practice is to specify the "noordered" clause for a sequence.  With a non-ordered sequence, a global lock not required by a node whenever you access the sequence.

  •  Increase sequence index freelist groups (or use ASSM):  Indexes with key values generated by sequences tend to be subject to leaf block contention when the insert rate is high.

    There are sources of contention in RAC with sequences.  Indexes are usually supported by indexes and it is not uncommon to witness index block leaf contention with high insert rates as many insert statements compete for the highest index leaf block in order to get their sequence number.

    One remedy is to alter the index to use additional freelists groups.  You can often achieve the same relief from index block contention by using ASSM or using ht sequence "cache" option", but beware that ASSM performance may degrade under "massive" insert DML. (over 100 inserts per second)

  •  Use sequence staggering:  A staggered sequence insert trigger is a specific constant added to the sequence value based on the instance number. This isolates each set of inserts and prevents inadvertent attempts to use the same sequence number.

These are the  most common method that are used to reduce RAC sequence contention.  For more details or to have a RAC expert certify your database for performance, consider using a remote RAC health check.

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