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Making
Sense of Storage Virtualization
Page 1, 2, 3,
4, 5
Appliance-based. Appliance-based
virtualization is the most recent approach
to enter the marketplace. It provides
customers with virtualization between
the hosts and the storage, allowing for the
same level of control and centralization
across the storage architecture. This
approach does not require dependencies
on the vendor, the presentation of
logical volumes, nor the operating system of
the open systems hosts. There are two kinds of
appliance-based virtualization products: in-band and out-of-
band.
An in-band virtualization appliance is
physically located between the host and
the storage. The appliance takes the
disk requests from the host and fulfills
the host’s request from the storage
attached to the other side of the
appliance. This functionality is essentially
transparent to the host because the
appliance presents itself as disk.
In-band virtualization reduces the amount
of administrative time needed to load
drivers onto each operating system
platform.
Since in-band virtualization’s
presentation of storage to the host is Indistinguishable
from that of a normal disk drive,
drivers need not be loaded on each
host. Figure 1 illustrates a
typical in-band, appliance- based
storage virtualization configuration.
There is a risk of overloading the
appliance if several hosts make
simultaneous disk requests.
Operators should consider redundancy–such
as implementing two appliances
rather than just one. This also
permits the operator to run appliances
in Active/Active mode.
For example,
Appliance A could be configured to
serve a certain pool of hosts and Appliance
B could be configured to serve a
different pool. In the event of a failure to
Appliance A, for example, Appliance B
could serve all hosts with a predictable effect
on performance.
The physical location of the
appliance is the primary
difference between out-of-band and
in-band appliances.
Out-of-band appliances logically
present themselves as if they are
located between the host and
storage, but they actually reside
to the side. This is accomplished with
the installation of a driver under the
host’s disk driver. The appliance driver then
receives logical to physical block
mappings from the appliance.
When the disk driver accesses the
disk, it’s actually accessing
the appliance’s driver. The
appliance driver then translates the
block information and accessing the
correct storage blocks. This functionality is
analogous to Network Address Translation
(NAT) except it is applied to
disks rather than networks. We call it Disk
Block Address Translation (DBAT).
Imation Storage Networking Lab
testing indicates that
out-of-band virtualization appears
to be a promising approach. Essentially,
out-of-band virtualization appliances
use pointers to manage the disk
pool. Because it has no direct interaction with
the host or the storage during I/O operations, the system delivers
nearly 95 percent of original disk
performance.
Figure 2 illustrates a standard
out-of-band virtualization
configuration.
Out-of-band, appliance-based
virtualization in a complex,
heterogeneous environment may not
yet be feasible due to limited
out-of-band appliance support for
the array of operating systems on the
market. As demand for out-of-band appliances
increases, however, we expect expanded
operating system support to emerge
soon.
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