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

Chapter 6, SFCFS Architecture
About CFS
59
SFCFS Backup Strategies
The same backup strategies used for standard VxFS can be used with SFCFS 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 snapshot file system for backups increases the load on the system because of
the resources used to perform copy-on-writes and to read data blocks from 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.
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 node.
◆ A snapshot is accessible only on the node mounting a 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 active on that device.
◆ On the node mounting a snapshot, the snapped file system cannot be unmounted
while the snapshot is mounted.
◆ A SFCFS 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
snapshot of a cluster file system only if the snapped cluster file system is mounted
with the crw option.