138 SANRAD V-Switch User Manual
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.
