Administrator's Guide

Cluster File System Administration
Snapshots for Cluster File Systems
Chapter 3
37
Snapshots for Cluster File Systems
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.
Snapshots implement copy-on-write semantics that incrementally copy data blocks when
they are overwritten on the “snapped” file system.
Snapshots for Serviceguard cluster file systems extend the same copy-on-write
mechanism for the I/O originating from any node in a CFS cluster.
Cluster Snapshot Characteristics
• A snapshot for a cluster mounted file system can be mounted on any node in a
cluster. The file system node can be a primary, secondary, or secondary-only node. 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 node, or on a
different node in a cluster.
• A snapshot is accessible only on the node it is mounted on. The snapshot device
cannot be mounted on two different 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
in a cluster 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. A snapshot is not affected if any other 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.
Performance Considerations
Mounting a snapshot file system for backup 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. Running a backup
application by mounting a snapshot from a lightly loaded node is beneficial to overall
cluster performance.
Creating a Snapshot on a Cluster File System
The following example shows how to create and mount a snapshot on a two-node cluster
using CFS administrative interface commands.
1. Create a VxFS file system on a shared VxVM volume:
# mkfs –F vxfs /dev/vx/rdsk/cfsdg/vol1