Veritas Storage Foundation 5.1 SP1 Cluster File System Administrator"s Guide (5900-1738, April 2011)

overall snapshot overhead. Therefore, running a backup application by mounting
a snapshot from a relatively less loaded node is beneficial to overall cluster
performance.
The following are several characteristics of a cluster snapshot:
■ A snapshot for a cluster mounted file system can be mounted on any node in
a cluster. The file system can be a primary, secondary, or secondary-only. A
stable image of the file system is provided for writes from any node.
See the mount_vxfs manual page for more information on secondary-only
(seconly) is a CFS mount option.
■ Multiple snapshots of a cluster file system can be mounted on the same or
different cluster nodes.
■ A snapshot is accessible only on the node mounting the snapshot. The snapshot
device cannot be mounted on two nodes simultaneously.
■ The device for mounting a snapshot can be a local disk or a shared volume. A
shared volume is used exclusively by a snapshot mount and is not usable from
other nodes as long as the snapshot is mounted on that device.
■ On the node mounting a snapshot, the snapped file system cannot be
unmounted while the snapshot is mounted.
■ A SFCFS snapshot ceases to exist if it is unmounted or the node mounting the
snapshot fails. However, a snapshot is not affected if another node leaves or
joins the cluster.
■ A snapshot of a read-only mounted file system cannot be taken. It is possible
to mount a snapshot of a cluster file system only if the snapped cluster file
system is mounted with the crw option.
In addition to frozen images of file systems, there are volume-level alternatives
available for shared volumes using mirror split and rejoin. Features such as Fast
Mirror Resync and Space Optimized snapshot are also available.
See the Veritas Volume Manager System Administrator’s Guide.
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.
27Storage Foundation Cluster File System architecture
Parallel I/O