Veritas Storage Foundation™ 5.0.1 for Oracle RAC Installation, Configuration, and Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

Access to cluster storage in typical SGeRAC configurations use CFS. Raw access to CVM volumes
is also possible but not part of a common configuration.
CFS Architecture
SGeRAC uses CFS to manage a file system in a large database environment. Since CFS is an
extension of VxFS, it operates in a similar fashion and caches metadata and data in memory
(typically called buffer cache or vnode cache). CFS uses a distributed locking mechanism called
Global Lock Manager (GLM) to ensure all nodes have a consistent view of the file system. GLM
provides metadata and cache coherency across multiple nodes by coordinating access to file
system metadata, such as inodes and free lists. The role of GLM is set on a per-file system basis
to enable load balancing.
CFS involves a primary/secondary architecture. One of the nodes in the cluster is the primary
node for a file system. Though any node can initiate an operation to create, delete, or resize data,
the GLM master node carries out the actual operation. After creating a file, the GLM master node
grants locks for data coherency across nodes. For example, if a node tries to modify a block in a
file, it must obtain an exclusive lock to ensure other nodes that may have the same file cached
have this cached copy invalidated.
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.
18 Introducing Serviceguard Extension for RAC