Administrator's Guide

Cluster File System Architecture
About CFS
Chapter 2
19
Cluster File System Backup Strategies
The same backup strategies used for standard VxFS can be used with CFS, because the
APIs and commands for accessing the namespace are the same. File system checkpoints
provide an on-disk, point-in-time copy of the file system. HP recommends file system
checkpoints over file system snapshots (described below) for obtaining a frozen image of
the cluster file system, because the performance characteristics of a checkpointed file
system are better in certain I/O patterns.
NOTE See the Veritas File System Administrator's Guide, HP-UX, 5.0 for a detailed explanation
and comparison of checkpoints and snapshots.
A file system snapshot is another method for obtaining a file system on-disk frozen
image. The frozen image is non-persistent, in contrast to the checkpoint feature. A
snapshot can be accessed as a read-only mounted file system to perform efficient online
backups. Snapshots implement “copy-on-write” semantics that incrementally copy data
blocks when they are overwritten on the “snapped” file system. Snapshots for cluster file
systems extend the same copy-on-write mechanism for the I/O originating from any
cluster node.
Mounting a snapshot filesystem for backups increases the load on the system because of
the resources used to perform copy-on-writes and to read data blocks from the snapshot.
In this situation, cluster snapshots can be used to do off-host backups. Off-host backups
reduce the load of a backup application on the primary server. Overhead from remote
snapshots is small when compared to overall snapshot overhead. Therefore, running a
backup application by mounting a snapshot from a relatively less loaded node is
beneficial to overall cluster performance.
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.