HP UX Serviceguard Storage Management Software manual Snapshots for Cluster File Systems

Cluster File System Administration

Snapshots for Cluster File Systems

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

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