As storage capacity continues to
grow at a compounded annual growth rate that exceeds 60%-90% for
Windows NT-demands for external storage under centralized management
persist. With these growing needs, continuous availability, ease
of management, scalability, and resource sharing-ultimately in
a heterogeneous environment-also receive increased emphasis. Two
specific architectures are prime candidates to fulfill these requirements:
Storage Area Networking (SAN) and Network Attached Storage (NAS).
Neither SAN nor NAS concepts are new, but their evpolution into the open system environments represents one of the most important changes affecting the storage industry in more than a decade. SAN and NAS are about to revolutionize the industry with ramifications extending beyond storage into network and systems management and architecture. SAN and NAS no longer couple storage directly to the server; this architectural change has important repercussions on the management of data in an enterprise.
What Is SAN?
No officially recognized definition for
SAN exists. The Storage Networking Industry Association (SNIA),
created in early 1998, is now defining SAN boundaries and establishing
standards to achieve interoperability. Until SNIA finishes, I
suggest that a SAN contains at least two servers with access to
a storage pool through an interconnection "fabric" with
at least one hub or switch. SANs also require network technologies
with high scalability, performance, and reliability to combine
the robustness and speed of a traditional storage environment
with the connectivity of a network. (To make sure you ask the
right questions on this complex subject, see "What to Look
For in an Enterprise SAN" on page XX).
A SAN is a specialized high-speed network that enables fast, reliable
access among servers and external or independent storage resources.
In a SAN, all networked servers share storage devices as peer
resources; they are not the exclusive property of any one server.
You can use a SAN to connect servers to storage, servers to each
other, and storage to storage through hubs, switches, and routers.
A SAN carries only I/O traffic between servers and storage devices;
it doesn't carry general-purpose traffic such as email or other
end-user applications. Thus, it avoids the difficult tradeoffs
inherent in using a single network for all applications.
As the SAN concept develops, it is growing beyond identification
with any one technology. Just as LANs use a diverse mix of technologies,
so do SANs. The SAN mix can include Enterprise Systems Connection
(ESCON), Fiber Distributed Data Interface (FDDI), Asynchronous
Transfer Mode (ATM), IBM's Serial Storage Architecture (SSA),
and Fibre Channel. SAN architectures also let you use a number
of underlying protocols, including TCP/IP and variants of SCSI.
The most popular implementation of SAN for open systems is based
on SCSI over Fibre Channel.
You can deploy SANs in both homogeneous and heterogeneous environments.
In a heterogeneous environment, a SAN allows different kinds of
servers-Windows NT, UNIX, and OS/390-to share different kinds
of storage-mainframe disk, tape, and redundant arrays of inexpensive
disks (RAID). With this shared capacity, organizations can acquire,
deploy, and use storage devices more cost-effectively. Ultimately,
on a SAN, any data at any network location is accessible, often
via multiple paths, by any nodes, applications, or users on the
network.
SAN Advantages
The first advantage of a SAN is superior connectivity. In a SAN
environment, every server on the network can address all the storage
on the network, for distances up to 10 km. with Fibre Channel
support. In fact, any storage device, if it has enough intelligence,
can talk to any other storage device on the network. This capability
enables the utilization of existing storage resources that may
be scattered throughout an enterprise building or campus, and
encourages broader adoption of remote vaulting, mirroring, and
clustering solutions.
SAN allows you to virtually centralize storage.
You can use SAN to isolate physical storage devices from the virtual
presentation that clients see. This separation provides a high
level of flexibility in distributing and reconfiguring resources.
Storage on a SAN is shared, resulting in centralized management,
better utilization of disk and tape resources, and enhanced enterprise-wide
data management and protection.
Other architectures can't match the degree of scalability that
SAN brings. Without affecting ongoing operations, storage capacity
can grow, its performance can scale along with its topology, and
its availability can increase by providing redundant systems and
data paths. SAN greatly improves performance. A large, switched
Fibre Channel fabric can sustain an aggregate bandwidth in the
gigabyte-per-second (GBps) range with very low latency.
What to Look For in an Enterprise SAN
In a series of recent surveys that Peripheral Concepts conducted,
administrators and managers of information systems were asked
to rank the characteristics they considered most important in
acquiring and managing their centralized storage. Five features-availability,
scalability, performance, connectivity, and manageability-consistently
ranked at the top of the most-wanted feature list. In preparation
for a SAN implementation, three additional characteristics are
also commonly expressed-security, interoperability, and end-to-end
support.
Availability: By far the most important user concerns are
full data and application accessibility. Beyond system and path
redundancy and failover, data availability implies serviceability,
component monitoring, automatic fault detection, isolation and
recovery, online repairability, and complete system restoration
after fixing a failure. Application availability means having
a thorough testing program that includes fast recovery from unexpected
conditions caused by system component failures. SAN will achieve
these availability goals via multiple levels of path and system
redundancy and powerful backup and disaster-recovery software.
Scalability: Scalability for a SAN must allow growth in
capacity, performance, connectivity, and availability without
affecting ongoing operations. Unlike classic clustering architecture,
a SAN configuration is expected to grow without adding complexity
and without performance degradation. Scalability includes adaptability
and portability-for products and applications.
Performance: Several levels of performance improvements
are expected from SAN, combining multiple simultaneous data paths,
load-balancing software, and dynamic path reallocation. Other
potential performance improvements relate to the design of admission-control
mechanisms guaranteeing bandwidth for high-priority data streams
and time-dependent events. Resource sharing, data sharing, and
Storage Resource Management (SRM) will increase overall performance.
Connectivity: Several aspects apply, ranging from tying
in new elements without disrupting ongoing operations, added distance,
and serving a variety of platforms or OSs. In a SAN environment,
every network server can address all the storage on the network.
As the technology matures, every storage device should be able
to talk to every other storage device on the network. This level
of connectivity will enable sharing business-critical data between
heterogeneous platforms.
Manageability: Managing storage is one of the most significant
costs in administering networks. Increased manageability has become
a priority in all distributed network environments. Data management
and SRM automate the processes of backup, restoration, configuration,
monitoring, load balancing, diagnosing, and reporting. The extent
to which a SAN can integrate and automate some or all of these
steps will determine its position in the hierarchy of product
classes.
Interoperability: A SAN contains many components, and the
lack of established standards has raised the interoperability
issue to a high level of concern. The requirement is for all these
elements to dialog and integrate harmoniously, to allow the user
the greatest flexibility in the choice of products and vendors.
The existing Fibre Channel standards are a prerequisite, but are
not sufficient to achieve interoperability among devices from
various vendors. In the interim period, the user will need the
vendor to guarantee and maintain interoperability. Working groups
within the Storage Networking Industry Association (SNIA) are
currently defining standards to alleviate this problem.
Security: In an early stage, the main security is integrated
in the server OS, but to ensure universal access, SAN management
must embed several levels of security. Early implementations address
access control through zoning and partitioning techniques
End-to-End Service Support: Switching to SAN is an important
undertaking, one that most users are not prepared to handle by
themselves. Vendors must help to plan, integrate, install, and
service the entire SAN infrastructure.
By: Farid Neema
PERIPHERAL CONCEPTS, INC.
351 Hitchcock Way, Suite #B-200
Santa Barbara, California, 93105
Tel: (805) 563-9491
fneema@silcom.com
This article was published in the May 1999 issue of Windows NT Magazine