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Making
Sense of Storage Virtualization
DIFFERENT APPROACHES
TO STORAGE VIRTUALIZATION
Based on our hands-on experience
with most of the virtualization
products in the marketplace
today, we categorize these
tools in three different ways—each is based
upon where the virtualization is executed:
• Host-based
• Storage-based
• Appliance-based
Host-based. Of
the different approaches to
virtualization, host-based virtualization was
the first to emerge, and as such
is the most widely used. Depending
on the operating system provider, host-based
virtualization is either packaged
with the operating system or
made available as an add-on product. At a high level, host-based virtualization al-lows an administrator to put one or more physical disks into a logical group and then split that group into one or more logical volumes. From there, data can be stored on those logical volumes as though they were physical disks of the specified size.
Host-based virtualization allows administrators to access advanced storage
management
functions such as mirroring,
RAID sets, redundant pathing, and hot
backups (by using mirror splits or snap-shots).
It adds some additional overhead to the
host system and the management of the
virtualization must be done on a host-by-host
basis, eliminating the possibilities
for global storage management. Further,
security and LUN masking must be handled
in the switches or storage subsystems.
As a result of their longer tenure in the
marketplace, host-based virtualization pro-ducts
are more mature than other types.
In addition, there are a greater number
of production implementations that provide
information on how the products
work and which pitfalls to avoid. However,
host-based virtualization is operating
system dependent. If a particular vendor
or third-party supplied package does not
exist for your selected operating system,
you can be left without options.
Storage-based. An
alternative to host-based allow for movement between different
RAID groups without data loss, as well
as automatic migration of data from
one RAID group to another based upon
the frequency of data access. In
addition, these products permit the creation
of multiple data mirrors, which provide
additional availability when one of the
mirrors is split for hot backups. One of
the most notable improvements is the
movement of these functions off the
host and onto the storage subsystem—
relieving the overhead on the host.
Storage-based virtualization also
introduced some new concepts, such as the
availability of storage beyond what host-based
virtualization provides, the
flexibility to modify LUN size, the ability to have
multiple hosts see the same LUNs (which
is particularly critical with high
availability clustering), and remote
replication.
Of course, these new concepts also
introduce new challenges.
Although storage-based virtualization
provides administrators with the ability
to modify LUN size within the storage
subsystem, the hosts connected to that
subsystem will still address that LUN as
if it were a physical disk. Consider, for
example, a situation in which the
administrator increases the size of a
particular LUN. Even though the LUN has been
resized, the host may continue seeing
the LUN at the original, smaller size.
Then consider a similar sizing challenge,
but with the LUN size decreased.
Although there are third-party packages
to tackle this very problem, users should
understand that this approach
introduces an additional layer of storage
management into their environment.
Next
Page >>Storage
Appliance Virtualization
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