VERITAS Storage Foundation 4.1 Cluster File System HP Serviceguard Storage Management Suite Extracts, December 2005

About CFS
58 Installation and Administration Guide
Mounting the primary with only the –o cluster,ro option prevents the secondaries
from mounting in a different mode; that is, read/write. Note that rw implies read/write
capability throughout the cluster.
Parallel I/O
Some distributed applications read and write to the same file concurrently from one or
more nodes in the cluster; for example, any distributed application where one thread
appends to a file and there are one or more threads reading from various regions in the
file. Several high-performance compute (HPC) applications can also benefit from this
feature, where concurrent I/O is performed on the same file. Applications do not require
any changes to use parallel I/O feature.
Traditionally, the entire file is locked to perform I/O to a small region. To support parallel
I/O, CFS locks ranges in a file that correspond to an I/O request. The granularity of the
locked range is a page. Two I/O requests conflict if at least one is a write request, and the
I/O range of the request overlaps the I/O range of the other.
The parallel I/O feature enables I/O to a file by multiple threads concurrently, as long as
the requests do not conflict. Threads issuing concurrent I/O requests could be executing
on the same node, or on a different node in the cluster.
An I/O request that requires allocation is not executed concurrently with other I/O
requests. Note that when a writer is extending the file and readers are lagging behind,
block allocation is not necessarily done for each extending write.
If the file size can be predetermined, the file can be preallocated to avoid block allocations
during I/O. This improves the concurrency of applications performing parallel I/O to the
file. Parallel I/O also avoids unnecessary page cache flushes and invalidations using
range locking, without compromising the cache coherency across the cluster.
For applications that update the same file from multiple nodes, the -nomtime mount
option provides further concurrency. Modification and change times of the file are not
synchronized across the cluster, which eliminates the overhead of increased I/O and
locking.
CFS Namespace
The mount point name must remain the same for all nodes mounting the same cluster
file system.