Veritas Storage Foundation 5.0 for Oracle RAC Configuration Guide Extracts for HP Serviceguard Storage Management Suite, Second Edition, May 2008

Introducing Serviceguard Extension for RAC
Cluster File System
Chapter 1
21
Cluster File System
CFS enables you to simultaneously mount the same file system on multiple nodes and is
an extension of the industry-standard Veritas File System. Unlike other file systems
which send data through another node to the storage, CFS is a true SAN file system. All
data traffic takes place over the storage area network (SAN), and only the metadata
traverses the cluster interconnect.
In addition to using the SAN fabric for reading and writing data, CFS offers storage
checkpoints and rollback for backup and recovery.
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.