READ ME before using the Veritas Storage Foundation™ 5.1 SP1 for Oracle RAC Administrator's Guide (April 2011)

SGeRAC configurations minimize the use of GLM locking. Oracle RAC accesses the file system
through the ODM interface and handles its own locking; only Oracle (and not GLM) buffers data
and coordinates write operations to files. A single point of locking and buffering ensures maximum
performance. GLM locking is only involved when metadata for a file changes, such as during
create and resize operations.
CFS Communication
CFS uses port f for GLM lock and metadata communication. SGeRAC configurations minimize the
use of GLM locking except when metadata for a file changes.
Oracle Disk Manager
SGeRAC requires Oracle Disk Manager (ODM), a standard API published by Oracle for support
of database I/O. SGeRAC provides a library for Oracle to use as its I/O library.
ODM Architecture
When the ODM library is linked, Oracle is able to bypass all caching and locks at the file system
layer and to communicate directly with raw volumes. The SGeRAC implementation of ODM
generates performance equivalent to performance with raw devices while the storage uses
easy-to-manage file systems.
All ODM features can operate in a cluster environment. Nodes communicate with each other before
performing any operation that could potentially affect another node. For example, before creating
a new data file with a specific name, ODM checks with other nodes to see if the file name is
already in use.
ODM Performance Enhancements
ODM enables the performance benefits provided by Oracle Disk Manager:
• Locking for data integrity.
• Few system calls and context switches.
• Increased I/O parallelism.
• Efficient file creation and disk allocation.
Databases using file systems typically incur additional overhead:
• Extra CPU and memory usage to read data from underlying disks to the file system cache.
This scenario requires copying data from the file system cache to the Oracle cache.
• File locking that allows for only a single writer at a time. Allowing Oracle to perform locking
allows for finer granularity of locking at the row level.
• File systems generally go through a standard Sync I/O library when performing I/O. Oracle
can make use of Kernel Async I/O libraries (KAIO) with raw devices to improve performance.
ODM Communication
ODM uses port d to communicate with other ODM instances to support the file management features
of Oracle Managed Files (OMF). OMF enables DBAs to set init.ora parameters for db datafile,
controlfile, and logfile names and for those structures to be named automatically. OMF allows for
the automatic deletion of physical data files when DBAs remove tablespaces.
Additional Features of Serviceguard Extension for RAC
Additional SGeRAC features include:
• The ability to transition from a local high-availability cluster into a disaster recovery environment.
This environment ensures maximum data protection and availability in the event of large-scale
14 Introducing Serviceguard Extension for RAC