The same backup strategies used for standard VxFS can be used with SFCFSHA 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. Because performance characteristics of a checkpointed file system are better in certain I/O patterns, they are recommended over file system snapshots (described below) for obtaining a frozen image of the cluster file system.
File System snapshots are another method of 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 of the file system. 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 from 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.
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.
See the mount_vxfs manual page for more information on secondary-only (seconly) is a CFS mount option.
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 mounting the snapshot. 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 mounted on that device.
On the node mounting a snapshot, the snapped file system cannot be unmounted while the snapshot is mounted.
A SFCFSHA snapshot ceases to exist if it is unmounted or the node mounting the snapshot fails. However, a snapshot is not affected if another 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 frozen images of file systems, 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.