Veritas Storage Foundation 5.0 Cluster File System Administration Guide Extracts for HP Serviceguard Storage Management Suite, Second Edition, May 2008

Technical Overview
Overview of Cluster File System Architecture
Chapter 1
8
Overview of Cluster File System Architecture
CFS allows clustered servers to mount and use the same file system simultaneously, as if
all applications using the file system are running on the same server. CVM makes logical
volumes and raw device applications accessible throughout a cluster.
Cluster File System Design
Beginning with version 5.0, CFS uses a Symmetric architecture in which all nodes in the
cluster can simultaneously function as metadata servers. CFS 5.0 has some remnants of
the master/slave node concept from version 4.1, but this functionality has changed in
version 5.0 along with a different naming convention. The first server to mount each
cluster file system becomes the primary CFS node; all other nodes in the cluster are
considered secondary CFS nodes. Applications access user data directly from the node
they are running on. Each CFS node has its own intent log. File system operations, such
as allocating or deleting files, can originate from any node in the cluster.
NOTE The master/slave node naming convention continues to be used when referring to Veritas
Cluster Volume Manager (CVM) nodes.
Cluster File System Failover
If the server designated as the CFS primary node fails, the remaining nodes in the
cluster elect a new primary node. The new primary node reads the intent log of the old
primary node and completes any metadata updates that were in process at the time of
the failure.
Failure of a secondary node does not require metadata repair, because nodes using a
cluster file system in secondary mode do not update file system metadata directly. The
Multiple Transaction Server distributes file locking ownership and metadata updates
across all nodes in the cluster, enhancing scalability without requiring unnecessary
metadata communication throughout the cluster. CFS recovery from secondary node
failure is therefore faster than from primary node failure.
Group Lock Manager
CFS uses the Veritas Group Lock Manager (GLM) to reproduce UNIX single-host file
system semantics in clusters. This is most important in write behavior. UNIX file
systems make writes appear to be atomic. This means that when an application writes a
stream of data to a file, any subsequent application that reads from the same area of the
file retrieves the new data, even if it has been cached by the file system and not yet
written to disk. Applications can never retrieve stale data, or partial results from a
previous write.
To reproduce single-host write semantics, system caches must be kept coherent and each
must instantly reflect any updates to cached data, regardless of the cluster node from
which they originate. GLM locks a file so that no other node in the cluster can
simultaneously update it, or read it before the update is complete.