As businesses grow and storage requirements spiral upward, storage
servers and arrays invariably proliferate throughout the
organisation. The presence of such disparate storage resources
eventually becomes difficult to manage, and this often leads to
underutilised (even lost) storage. In fact, it's common to see
storage utilisation at only about 50% -- resulting in wasted
capital expenditures on new storage.
Storage virtualisation has emerged as an answer to this
dilemma, allowing storage administrators to identify, provision and
manage disparate storage as a single aggregated resource. This is
an important element of
storage consolidation, easing management headaches and allowing
higher levels of storage utilisation, which in turn forestalls the
expense of added storage. Let's examine some of the key issues
involved with storage virtualisation.
Important pros and cons
Storage virtualisation basically works by adding a layer of
abstraction (typically software) between the storage systems and
the applications that use storage. Applications no longer need to
know what disks, partitions or storage subsystems their data is
stored on. When implemented properly, storage utilisation can
improve to 80% or better. Virtualisation technology can improve
availability. For example, an application may be associated with
specific storage resources, and any interruption to those resources
will adversely affect the application's availability. With storage
virtualisation, the application is no longer coupled to the
physical implications of storage.
Storage virtualisation can help to automate storage capacity
expansion. Instead of manual provisioning, virtualisation can apply
policies that assign more capacity to applications as needed. In
addition, storage virtualisation can also allow storage resources
to be altered and updated on the fly without disrupting application
performance, generally reducing storage downtime for repairs and
maintenance.
The problem with this is complexity -- the virtualisation layer
is one more element of the storage environment that must be managed
and maintained as virtualisation products are patched and updated.
It's also important to consider the impact of storage
virtualisation on interoperability and compatibility between
storage devices. In some cases, the virtualisation layer may
potentially interfere with certain special features of storage
systems, such as remote replication.
Another issue with storage virtualisation is the difficulty
involved with undoing or "backing out" once virtualisation has been
implemented. It's not impossible, but the process of reassociating
applications with storage locations can be a painful process.
Consequently, experts suggest implementing virtualisation in a
piecemeal fashion -- starting with a limited deployment and then
systematically building out across the data center and the entire
organisation.
Finding the right virtualisation point
Storage virtualisation can be implemented at the host level, the
network level or the storage system level. Host-based
virtualisation is the easiest and most straightforward out-of-band
method, but it scales poorly and maintaining virtualisation servers
can be troublesome, especially if an agent must be installed and
maintained on each virtualised storage device. Conversely, storage
virtualisation can be accomplished in the storage array itself
(e.g. a TagmaStore system from Hitachi Data Systems). This offers
convenience, but such vendor-centric deployment is generally not
heterogeneous.
Today, the most popular point of implementation for storage
virtualisation is in the network fabric itself -- often through a
dedicated virtualisation appliance or an intelligent switch running
virtualisation software, such as IBM's SVC software. Network-based
storage virtualisation is the most scalable and interoperable point
of deployment -- making it particularly well suited to storage
consolidation projects -- but there may be a slight impact on
network performance due to in-band processing in the virtualisation
layer.
Influence on disaster recovery
Storage virtualisation can also affect consolidation in backups
and disaster recovery (DR). In many cases, replication, especially
remote replication, takes place between two identical storage
systems (e.g., Symmetrix to Symmetrix) so that the duplicate data
maps exactly to the original storage system(s). By virtualising
storage, data can be replicated to almost any storage hardware at
the disaster recovery site. This is often beneficial when older
storage hardware is displaced by newer systems. The older hardware
can then be redeployed at the disaster recovery site and continue
to serve a valuable function.