Veritas Storage Foundation 5.0.1 Cluster File System Administrator's Guide Extracts for the HP Serviceguard Storage Management Suite on HP-UX 11i v3

There are several characteristics of a cluster snapshot, including:
• 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.
• 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 it is mounted on. 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 active on that device.
• On the node mounting a snapshot, the snapped file system cannot be unmounted while the
snapshot is mounted.
• A CFS snapshot ceases to exist if it is unmounted or the node mounting the snapshot fails.
However, a snapshot is not affected if a 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 file-level frozen images, 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.
Synchronizing Time on Cluster File Systems
CFS requires that the system clocks on all nodes are synchronized using some external component
such as the Network Time Protocol (NTP) daemon. If the nodes are not in sync, timestamps for
creation (ctime) and modification (mtime) may not be consistent with the sequence in which
operations actually happened.
Distributing Load on a Cluster
For example, if you have eight file systems and four nodes, designating two file systems per
node as the primary is beneficial. The first node that mounts a file system becomes the primary
for that file system.
You can also use the fsclustadm command to designate a CFS primary. The fsclustadm
setprimary command can be used to change the primary. This change to the primary is not
persistent across unmounts or reboots. The change is in effect as long as one or more nodes in
the cluster have the file system mounted. The primary selection policy can also be defined by an
HP Serviceguard attribute associated with the CFS mount resource.
File System Tuneables
Tuneable parameters are updated at the time of mount using the tunefstab file or vxtunefs
command. The file system tunefs parameters are set to be identical on all nodes by propagating
the parameters to each cluster node. When the file system is mounted on the node, the tunefs
parameters of the primary node are used. The tunefstab file on the node is used if this is the
first node to mount the file system. HP recommends that this file be identical on each node.
Single Network Link and Reliability
In some environments, you may prefer using a single private link, or a pubic network, for
connecting nodes in a cluster - despite the loss of redundancy if a network failure occurs. The
benefits of this approach include simpler hardware topology and lower costs; however, there is
obviously a tradeoff with high availability.
18 Cluster File System Architecture