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8-2 SANRAD V-Switch CLI 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.