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

Cluster File System Architecture
About CFS
Chapter 2
17
About CFS
If the CFS primary node fails, the remaining cluster nodes elect a new primary node.
The new primary node reads the file system intent log and completes any metadata
updates that were in process at the time of the failure. Application I/O from other nodes
may block during this process and cause a delay. When the file system becomes
consistent again, application processing resumes.
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.
See “Distributing Load on a Cluster” on page 20.
Cluster File System and The Group Lock Manager
CFS uses the Veritas Group Lock Manager (GLM) to reproduce UNIX single-host file
system semantics in clusters. UNIX file systems make writes appear atomic. This means
when an application writes a stream of data to a file, a subsequent application reading
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 cannot retrieve stale data or partial
results from a previous write.
To simulate single-host write semantics, system caches are kept coherent and each
node’s cache instantly reflects updates to cached data, regardless of the node from which
the update originates.
Asymmetric Mounts
A Veritas™ File System (VxFS) mounted with the mount -o cluster option is a cluster or
shared mount, as opposed to a non-shared or local mount. A file system mounted in
shared mode must be on a Veritas™ Volume Manager (VxVM) shared volume in a cluster
environment. A local mount cannot be remounted in shared mode and a shared mount
cannot be remounted in local mode. File systems in a cluster can be mounted with
different read/write options. These are called asymmetric mounts.
Asymmetric mounts allow shared file systems to be mounted with different read/write
capabilities. One node in the cluster can mount read/write, while other nodes mount
read-only.
You can specify the cluster read-write (crw) option when you first mount a file system, or
the options can be altered when doing a remount (mount -o remount). The first column
in Table 2-1 on page 18 shows the mode in which the primary node is mounted. The X
marks indicate the modes available to secondary nodes in the cluster.
See the mount_vxfs(1M) manual page for more information.