virtualization is often associated with SANs its benefits
are not new and can also be realized in 'non-SAN' environments.
by Savio Monteiro
virtualization is yet another example of the old becoming
new. Although the concept is finding new life and importance
with the emergence of SANs, storage virtualization technologies
are widely used in both mainframe and open systems environments
to help simplify administration and provide flexibility
in demanding storage environments. Amidst all the excitement
about SAN virtualization, it's easy to lose sight of
the benefits of storage virtualization in the prevailing
DAS (Direct Attached Storage) architecture.
Despite the hype and new product introductions, SANs
have not yet been widely adopted in corporate IT. The
Enterprise Storage Group estimates that in large U.S.
companies, only about 5 percent of storage is in a Fiber
Channel SAN. The remaining 95 percent is DAS. If you're
handling that 95 percent today, storage virtualization
can offer immediate relief in terms of storage manageability
combines multiple physical storage devices into
logical (virtual) storage devices or units
the recent surge in interest, storage virtualization
is not new, either in concept or in practice. Defined
almost 20 years ago in a technical white paper created
by an IBM mainframe users' group, the concept of storage
virtualization was put into practice in the MVS operating
system so that strings of DAS could be addressed as
simpler, logical entities.
Storage virtualization is the process of taking multiple
physical storage devices and combining them into logical
(virtual) storage devices or units that are presented
to the OS, applications, and users. In a sense, storage
virtualization builds a layer of abstraction above the
physical storage (see diagram).
The important part
important part is what can happen in this abstraction
process. Since data is not tied to specific hardware
devices, virtualization provides a very flexible storage
environment. It simplifies the management of storage
and can potentially reduce costs through better hardware
utilization and consolidation.
The virtual devices are not restricted by the capacity,
speed, and reliability limitations of the physical devices.
By applying intelligent storage software in the virtualization
layer, virtualization offers a way to address the functional
challenges of storage.
Users are not generally interested in the physical aspects
of the storage serving their applications. They don't
want to hear about seek times or rotational latency.
They don't care how many disks are in a string or the
mean time between failure for those disks.
What they do care about are issues of application response
time and throughput, sufficient capacity for their data
as it grows, and application downtime. In short, they
care about the application aspects of their data, not
the physical aspects of storage.
The virtualization layer offers a chance to combine
physical devices into virtual entities that meet application
requirements. For example, you can create a device that
optimizes performance for a specific application, while
shielding users and applications from the physical details
of the implementation. As new hardware becomes available
or application characteristics change, you can modify
the physical layer without interrupting access to the
One of the benefits of storage virtualization, particularly
in a SAN environment is that it enables consolidation,
which simplifies management and possibly reduces the
total amount of storage managed.
Some organizations create SLA (Service Level Agreements)
with their constituents which promise specific levels
of availability and performance. Administrators, often
with their jobs on the line, have to operate conservatively.
They would rather over-provision than under-provision
storage for a critical application. This leads to an
underutilization of storage. According to Forrester
Research, many organizations use only about 50 percent
of their disk space.
At the same time, they may buy proprietary storage systems
that promise certain levels of availability or performance
and then find themselves with additional management
tasks for those proprietary storage systems. All of
this can lead to a complex administrative environment.
Logical volume manager
In an open systems environment, logical volume managers
virtualize storage by consolidating physical storage
into logical volumes, which are available to applications
or file systems. Logical volume managers are available
on most Unix platforms (including Linux) and on Windows
By assembling virtual storage volumes out of numerous
physical devices, you can create storage configurations
tuned for specific applications, without the limitations
of specific hardware devices. Instead, you can make
use of whatever storage is at hand, without locking
into proprietary storage solutions.
Logical volume managers improve application availability
by building redundancy into the logical volume itself.
The possibilities go beyond simply mirroring and RAID.
For example, the failure of a device in a redundant
storage configuration can degrade performance and expose
the data to risk from another failure. The logical volume
manager can maintain a pool of spare disks that can
be automatically swapped in (hot relocation) when a
device fails in a logical volume. It can even move data
back automatically when the failed device is replaced
Unlike a physical device, a logical volume has a nearly
limitless capacity: Administrators can add storage as
needed without interrupting access to the data. When
used with a database's auto-extend capabilities or a
file system's automatic extension, a logical volume
manager can significantly ease the problem of provisioning
storage for growing applications.
Beyond the single system
The potential benefits of storage virtualization can
easily extend beyond any single system's needs. Since
individual administrators typically manage multiple
systems, administration can become a bottleneck in terms
of storage scalability.
The cost of managing storage is non-trivial, and over
time, exceeds the purchase price of the storage hardware.
One way to monitor this cost is to evaluate the amount
of storage managed per administrator. Efficient organizations
may have one administrator per terabyte of storage or
better, while others with more difficult environments
may have much lower numbers. Storage virtualization
can simplify the management of storage in heterogeneous
environments, ultimately reducing the true cost of the
Storage virtualization through logical volume management
is simply software. Although it needs to cooperate closely
with the host OS there is no reason why it should be
tied to any specific OS platform or storage device.
A logical volume manager should have cross-platform
capabilities and be hardware-agnostic, so organizations
can support heterogeneous environments and mix-n-match
storage as needed.
A cross-platform storage virtualization solution offers
distinct advantages for administrators or organizations
handling multiple OSs like a single interface for managing
storage across platforms and devices, centralized management
of distributed storage resources, and online administration.
Armed with these tools, organizations can create consistent
business methodologies for managing application storage,
regardless of platform. For example, companies using
virtualization for database storage can define corporate-level
policies for the logical volumes and file systems that
contain database data and components, mandating certain
levels of redundancy and performance.
At the same time, the separation of the logical and
physical provides flexibility to change or reconfigure
storage hardware transparently, perhaps swapping in
higher-performing devices or moving storage to the systems
that most need it, without interrupting access to the
So, storage virtualization, even in the DAS, open systems
environment, helps IT organizations achieve the SLAs
they commit to, while simplifying system management
and support growing heterogeneous systems. And this
kind of virtualization can be extended to shared-storage
Availability clusters that share a connection to the
same logical volumes, but 'roll over' ownership when
a primary server fails.
Shared data clusters in which multiple servers share
access to data residing on a single virtual volume.
Web servers accessing a common copy of a site is a
one good example of this kind of cluster. Clusters
can share data either through simple switched SCSI
devices (for smaller clusters) or through a SAN.
NAS devices hosting file systems that may be shared
among multiple servers.
Savio Monteiro is Country Sales Manager,