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 GLM revokes all shared locks before granting the exclusive lock and informs reading nodes that their data is no longer valid.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.