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

Chapter 7, SFCFS Administration

Snapshots on CFS

Using GUIs

Use the VERITAS Enterprise Administrator (VEA) for various VxFS functions such as

making and mounting file systems, on both local and cluster file systems.

Snapshots on CFS

A snapshot provides a consistent point-in-time image of a VxFS file system. A snapshot

can be accessed as a read-only mounted file system to perform efficient online backups of

the file system. Snapshots implement copy-on-write semantics that incrementally copy data

blocks when they are overwritten on the snapped file system (see the VERITAS File System

Administrator’s Guide). Snapshots for cluster file systems extend the same copy-on-write

mechanism for the I/O originating from any node in the cluster.

Cluster Snapshot Characteristics

1. 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.

2. Multiple snapshots of a cluster file system can be mounted on the same or a different

node in a cluster.

3. A snapshot is accessible only on the node mounting a snapshot. The snapshot device

cannot be mounted on two different nodes simultaneously.

4. 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 in

a cluster as long as the snapshot is active on that device.

5. On the node mounting a snapshot, the snapped file system cannot be unmounted

while the snapshot is mounted.

6. A CFS snapshot ceases to exist if it is unmounted or the node mounting the snapshot

fails. A snapshot, however, is not affected if any other node leaves or joins the cluster.

7. A snapshot of a read-only mounted file system cannot be taken. It is possible to mount

snapshot of a cluster file system only if the snapped cluster file system is mounted

with the crw option.