VERITAS Storage Foundation 4.1 for Oracle RAC HP Serviceguard Storage Management Suite Extracts, December 2005

Cluster File System
Cluster File System
CFS is an extension of the industry standard VERITAS File System (VxFS). CFS enables
you to simultaneously mount the same file system on multiple nodes. Unlike other
clustered file systems, CFS is a true SAN file system; all I/O occurs over the storage area
network. Coordination between nodes occurs through messages across the cluster
interconnects.
CFS Architecture
CFS incorporates a primary/secondary architecture. Though any node can initiate an
operation to create, delete, or resize data, the master node carries out the actual operation.
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). A distributed locking
mechanism called Global Lock Manager (GLM) provides metadata and cache coherency
across multiple nodes. GLM offers a way to ensure all nodes have a consistent view of the
file system. When any node wishes to read data, GLM requests a shared lock. If another
node wishes to write to the same area of the file system, it must request an exclusive lock.
GLM revokes all shared locks before granting the exclusive lock and informs reading
nodes that their data is no longer valid.
CFS Usage in Storage Foundation for Oracle RAC
Storage Foundation for Oracle RAC uses CFS to manage a file system in a large database
environment. Oracle uses the ODM interface to access data files stored on CFS file
systems. This process essentially bypasses the file system buffer and locking for data; only
Oracle (and not GLM) buffers data and coordinates writing to files. A single point of
locking and buffering ensures maximum performance.
Oracle Disk Manager
The Oracle Disk Manager (ODM) is a standard API specified by Oracle for database I/O.
When Oracle performs a write operation, it calls the odm_io function. ODM improves
both performance and manageability of the file system.
The VERITAS implementation of ODM improves performance by providing direct access
for the database to the underlying storage without passing through the actual file system
interface. The resulting performance is equivalent to performance when using raw
devices. The administrator sees the storage as easy-to- manage file systems that support
the resizing of data files while in use.
Chapter 1, Overview: Storage Foundation for Oracle RAC 9