Veritas Storage Foundation 5.1 SP1 Cluster File System Administrator"s Guide (5900-1738, April 2011)
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 Storage Foundation Advanced Features 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
A cluster snapshot has the following characteristics:
■ 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 a
different node in a cluster.
■ A snapshot is accessible only on the node mounting a snapshot. 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 SFCFS 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.
■ 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.
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 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.
Administering Storage Foundation Cluster File System and its components
Administering CFS
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