Oracle Tips  by Burleson Consulting

The Data Warehouse Development Life Cycle

Parallelism And Oracle Data Warehousing
The Distributed Lock Manager

Many parallel server novices are confused by the Distributed Lock Manager (DLM) and believe that the DLM is a component of the Oracle manufactured software. In reality, the DLM is a non-Oracle locking facility that is used in conjunction with the Oracle parallel server to synchronize concurrent processes. The DLMs are operating-system specific, and there are numerous DLM vendors for Solaris, AIX, and HP-UX implementations of Unix. At the most basic level, the DLM keeps track of the concurrent Oracle tasks executing on each CPU and keeps track of lock requests for resources.

With Oracle parallel server, the limitation of one instance to one database has been lifted. You can now configure many Oracle instances to share the same data warehouse. Of course, all of the Oracle instances are running in the same shared memory, and the DLM is used to provide locking between instances.

Note: You can tell if your system is running the DLM by checking for the LCK process. Just as the RECO process indicates that distributed transactions are enabled, the presence of an LCK process indicates that the DLM is active. Traditional distributed systems do not have a DLM because they are not sharing the same databases.

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This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm