SANRAD V-Switch manual Introduction to Advanced Volume Configurations, Mirror versus Snapshot

Introduction to Advanced Volume

Configurations

The V-Switch supports several advanced volume operations. Some do the same or similar functions. Each has its own advantages so it is important to understand their differences to best choose the function most appropriate for you SAN.

Data Replication: Off-line versus On-line

Off-line data replication creates a copy of a volume. The source volume can be any type of volume. If the source volume is exposed, it must be taken offline, creating an interruption of service to the host. The destination volume must also be off-line. On-line data replication is done via a mirrored volume. The source and destination volumes must be children the same mirror. On-line data replication presents no interruption of service. Because on-line replication must take into account the possibility of read/write operations to the source volume during the data replication, it is slower than off-line data replication.

Mirror versus Snapshot

A mirrored volume copy is a full, complete volume copy. A snapshot is only a record of changes to a volume. Because of this, its capacity can be smaller than a mirrored volume copy by as much as eighty percent. Both a mirrored volume copy and a snapshot can be exposed to a host like any other volume. However, unlike a mirrored copy, a snapshot is nonfunctional if its source volume goes off-line or its capacity is exceeded in write operations. A snapshot volume must also be exposed on the same V- Switch as its source volume while a mirrored copy has no dependency on its source.

Actual Capacity versus Potential Capacity

Potential capacity is the maximum capacity a volume could be. Actual capacity is the capacity that the server recognizes a virtual volume as having.

Resizing versus Expanding

Resizing increases a virtual volume’s potential capacity. Expanding increases a virtual volume’s actual capacity. A volume must first be resized. Only after a volume is resized can it be expanded.

138 SANRAD V-Switch User Manual