Storage Area Networks For Dummies®

Chapter 15: Everything You Ever Wanted to Know about Virtualization
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Few virtualization solutions are available with the capability to directly map
a physical disk as a virtual disk. The Hitachi Universal Storage Platform
(USP) can do it when attaching a LUN from a virtualized external array. There
would be no need to migrate any data from the external array, since all the
LUNs could be removed from the hosts, reattached behind the array, and
then zoned back in with the hosts. Although the LUNs would now look like
Hitachi LUNs, all the data would still be there, and there would be limited
changes to the host applications.
This next paragraph is a bit technical but important. By using a direct-mapping
virtualization solution, you can even preserve the SCSI inquiry string (the command
a server uses to ask the disk array what type of drives they are) so that
the hosts will see not only the same data but the same drive type. The only
thing that changes is the port connected to the fabric. All the virtual capabilities
are present but nothing changes on the original disk. (See Figure 15-11.)
The virtualization solution uses direct mapping (also known as service enabling)
of a physical LUN to a virtual LUN, so the data already on the LUN does not have
to be migrated into a storage pool and can stay where it is. The SCSI inquiry
string is preserved, so the host also still sees the LUN as an IBM LUN, which
further simplifies introducing virtualization into the SAN. The LUN now gains all
the benefits of being virtualized, such as data replication and mirroring, without
the risks.
Figure 15-11:
Direct
mapping
using a
solution that
preserves
the original
identity of
the LUN
being
virtualized.
Fabric
IBM
Original configuration:
Server sees IBM LUN
Fabric
IBM
Virtual Direct Mapping: Server
access to data through
virtualization appliance.
Server still sees IBM LUN.