is a SAN?
A storage area network (SAN)
is a high-performance subnet, probably (but not necessarily) based on fibre
channel, whose primary purpose is the transfer of data between computer systems
and storage elements and among multiple storage elements. One can think of a SAN
as an extended and shared storage bus. A SAN consists of a communication
infrastructure, which provides physical connections, and a management layer,
which organizes the connections, storage elements, and computer systems so that
data transfer is secure and robust.
are the Market Drivers for SANs?
The limitations in speed,
distance, and connectivity of SCSI technology prompted the search for an
alternative method to access storage devices. The need for LAN-free backups and
data sharing started the initial move toward SAN technology. These necessities
and the desire to keep all data on-line and accessible 24 hours a day to an
increasingly global and/or Internet-based user population are driving current
Backup Capacity: Increasing data storage requirements and the need
for 100% availability of applications have overwhelmed SCSI backups across
Capacity Growth: IDC and Gartner Group estimate that data is
growing at a rate of over 85% annually. To put this in perspective a 750GB
System Flexibility/Cost: SANs are storage-centric networks that
provide easy scalability, allowing servers or storage to be added
independently of each other.
Availability/Performance: The use of a storage data transmission
protocol like SCSI permits the transfer of large amounts of data with
limited latency and overhead.
are Storage Area Networks Needed?
Traditional means for connecting
servers and storage can no longer satisfy today's requirements for fast access
to massive amounts of data. SCSI technology uses parallel cabling which severely
limits speed, distance and the number of attached storage devices. Configuring
SCSI (Small Computer Systems Interface) connections to support terabytes of data
is impractical. In addition, traditional server/storage connections make the
server the exclusive owner of its attached storage. As computing environments
move from a server centric to data centric model, access to shared data
resources becomes critical. Storage Area Networks are an enabling technology
that allow storage resources to be shared in order to provide continuous,
faster, easier access to data.
are the Benefits of a SAN?
The primary benefits of a SAN are:
Availability: A single copy of data is accessible to any and all
hosts via multiple paths.
Reliability: Dependable data transportation ensures a low error
rate, and an ability to recover from failures is provided.
Scalability: Servers and storage devices may be added independently
of one another, and do not depend on proprietary systems.
Performance: Fibre Channel (the standard method for SAN
interconnectivity) has a 100MB/sec bandwidth and low overhead, and it
separates storage and network I/O.
Manageability: Emerging software and standards for both FC-AL and
Fibre Channel fabric allow single centralized management and proactive error
detection and correction.
Return On Information Management: Due to increased redundancy and
superior manageability, as well as the ability to add storage and servers
independently of one another SANs provide a lower cost of ownership and a
Higher Return On Information Management (ROIM).
should I use a Switch versus a Hub?
Hubs: Hubs are perfect for small, entry-level environments and
systems. The typically cost less and offer a lower throughput the switches.
Switches: Data-intensive, high-bandwidth
applications such as backup, video editing, and document scanning can make
full use of switches. Due to their redundant data paths and superior
manageability switches are also perfect when high availability is required.
there reasons to use Switches instead of Hubs in a SAN?
Switches provide several advantages in a SAN environment.
Failover Capabilities: In
a switched fabric, in the event that a single switch fails, other switches
in the fabric remain operational. A Hub based environment typically fails if
a single hub on the loop fails.
Switches support the Fibre Channel Switch (FC-SW) standard, which makes
addressing independent of the subsystem's location on the fabric and
provides superior fault isolation and high availability. FC-SW also allows
host to better identify subsystems connected to the switch.
Switches offer "multiple-transmission data flow", in which each
fabric connection can simultaneously maintain a 100MB/sec throughput. A hub
offers a single data flow with an aggregate throughput of 100MB/sec.
Interconnection switches provide thousands of connections without degrading
bandwidth. A hub-based loop is limited to 126 devices.
support the on-line addition of subsystems (servers or storage) without for
re-initialization or shutdown. Hubs require a Loop Initialization (LIP) to
reacquire subsystem addresses whenever changes occur on the loop. A LIP
typically takes 0.5 seconds and can disable a tape system that is in the
process of doing a backup.
does a SAN differ from a LAN or WAN?
A SAN is similar to a LAN, in that it is a methodology of connecting systems
together with standardized hardware and software protocols. A SAN differs from a
LAN is two main ways.
Storage versus Network
Protocol: A LAN uses network protocols that send smaller
"chunks" of data with increased communication overhead. This
reduces bandwidth. A SAN uses storage protocols (SCSI) that sends larger
"chunks" of data with reduced overhead and increased bandwidth.
Server Captive Storage:
LAN based systems connect servers to clients, with each server owning and
controlling access to its own storage resources. Storage must be added to a
server rather than directly to the LAN. A SAN allows storage resources to be
added to the network enabling any server to directly access storage
do I manage a SAN?
There are two basic methods for SAN management.
SNMP (Simple Network
Management Protocol): SNMP is based on TCP/IP and offers basic alert
management. This allows a node to alert the management system of failures
(drive, fan, power….). However SNMP does not offer proactive management
and lacks security.
Protocol: The second way to manage a SAN is to use a proprietary
management protocol. There are a number of manufacturers who provide SAN
management software (see SAN Management). In either case to run management
software typically requires a separate terminal (such as an NT server)
connected to the SAN. This enables additional capabilities, such as zoning
(security), mapping, masking, as well as backup and restore and fault
is a SAN Manager?
A San Manager is a proprietary Storage Area Network management software, which
enables management of fibre channel attached storage in a highly efficient
manner as an administrative cluster. SAN Managers enable systems to use a common
pool of storage devices on a Storage Area Network. Using a SAN Manager,
administrators can assign storage from pools of available capacity when and
where it is needed.
are the major SAN vendors?
Many key industry players support the evolution of SANs and are involved in the
development of the industry. These companies include ITIS Services, Hitachi Data
Systems, Hewlett-Packard, Compaq, StorageTek, Brocade, Gadzoox, Veritas, Legato,
Computer Associates, Oracle, DataCore, Ancor, Vixel, Sun, Dell and a number of
other industry leaders. The most active of these leaders have established
themselves as members of the Storage Network Industry Alliance (see SNIA).
is Fibre Channel?
Fibre Channel is a standards-based, gigabit transport that is optimized for
storage and other high-speed applications. Currently implemented in 1 gigabit
(200MBps full duplex) speed, Fibre Channel will support up to 400MBps full
duplex transfer speeds in the near future. There are three topologies based on
Fibre Channel - point to point, arbitrated loop and fabric. Arbitrated loop and
fabric provide the underlying infrastructure of most SAN implementations today.
Fibre Channel supports high speed transport, long distances (up to 60 km), and
up to 16 million devices in extended network configurations. By introducing the
scalability and flexibility of networking to the server/storage relationship,
Fibre Channel is enabling new storage applications for clustering, disaster
recovery, and shared resource requirements.
is Guiding the Development of Fibre Channel Standards?
Fibre Channel standards have been developed through ANSI (American National
Standards Institute) committees. Vendors and end-users are also represented by
industry bodies such as the Fibre Channel Industry Alliance (FCIA) and the
Storage Networking Industry Alliance (SNIA). Cooperative efforts by vendors and
customers are driving both standardization and interoperability to streamline
the adoption of SANs. ITIS Services is a voting member of the SNIA and an active
participant in today's industry standards.
is Fibre Channel "Fabric"?
In the early days of Fibre Channel, the concept of a universal
"Fabric" was popular as a means of supporting Fibre Channel's topology
independence. Its value has since been enhanced with the advent of point to
point and FC-AL topologies. The Fibre Channel Fabric was designed as a generic
interface between each node and the physical layer interconnect of these nodes.
By adhering to this interface, any Fibre Channel node could communicate over the
"Fabric" without being required to have specific knowledge of the
interconnecting scheme between nodes. A "Fabric" is similar to a
telephone switch in which we make a call and the party at the other end answers,
all without having detailed knowledge of the interconnection method between the
is Fibre Channel Arbitrated Loop?
Arbitrated loop is a shared architecture, which supports 100MBps or 200MBps full
duplex speed. Analogous to token ring, multiple servers or storage devices can
attach to the same loop segment. Up to 126 devices may be attached to a FC-AL,
although the majority of arbitrated loops are deployed with from 4 to 30
devices. Since the loop is a shared transport, devices must arbitrate for access
to the loop before sending data. Fibre Channel provides a superset of commands
to provide orderly access and ensure data integrity.
is a GBIC?
A GBIC is a gigabit interface converter. A removable transceiver commonly used
in Fibre Channel switches, hubs and host bus adapters. A transceiver converts
one form of signal into another or from fibre optic signals to electrical
signals. A critical component of high speed data transfer, GBIC's ensure the
quality of signaling and data integrity.
is an Interconnect?
Capable of speeds 100 times faster than current networks, Interconnect is the
pipeline used for high-speed, high-bandwidth data access within a storage area
network. It connects all of the pieces of a SAN and provides scalability,
connectivity, performance and availability.
is The Difference Between SAN and NAS?
Storage Area Networks (SAN) and Network-attached Storage (NAS) are similar, in
that they both represent a convergence of storage and networking technologies.
However, they are also entirely different. NAS products, such as Network
Appliance Filers and Auspex servers are storage devices fronted by a thin server
client that is directly attached to the messaging or public network. These
products tend to be optimized for file serving purposes only. Storage Area
Networks are separate networks dedicated to storage devices and traffic. SANs
provide greater flexibility and additional functionality over a NAS. However
each of these approaches has their merits, it is generally agreed that SANs
represent the future of storage connectivity. NAS devices will continue to
perform their specific functions but shall migrate, over time, to the Storage
Area Network model.
Quick will the SAN Market Develop?
Industry projections vary, and some are quite optimistic. According to
Dataquest, an industry watchdog, the SAN marketplace will grow 89% compounded
annually between 1999 and 2003. According to IDC SANs are expected to grow
ten-fold by 2002. To put a dollar figure on this, according to IDC the overall
disk storage market for the year 2000 will exceed $80 billion dollars.
do SCSI Tape Drives Connect to A Fibre SAN?
Fibre Channel to SCSI bridging products enable connection of SCSI tape
drives/libraries, or SCSI disk subsystems, to Fibre Channel-based SANs.
With any emerging technology, prospective end users need to be informed about
how, what and when standards will be adopted and approved by industry
manufacturers and developers. Armed with this knowledge, IT decision makers may
begin to assess where SAN's can be safely deployed throughout the enterprise
without risking loss of infrastructure investment. The SAN industry is
represented by several hardware, software and service consortiums. These
include: SNIA (Storage Network Industry Alliance), FCIA (Fiber Channel Industry
Association) and the FCA (Fiber Channel Alliance). In an effort to promote and
adopt universal SAN standards, these groups co-develop hardware and software
protocols. Many vendors within the industry are working towards an
"open" solution to enable end users the flexibility to mix and match
products and achieve a true "best of breed" data storage processing
environment. Several switch manufacturers, for example, are working to ensure
that their products can co-exist within a SAN. This is a key objective since
proprietary solutions may effectively forfeit the greatest possible return on
information management (ROIM) and lock users out from readily exploring
alternatives. The SAN industry has universally adopted fiber channel arbitrated
loop (FC-AL) as a protocol for exchanging data within switched and non-switched
environments. Switched fabric, on the other hand, has not emerged with a
complete set of standards agreed on by the industry. While there are successful
installations of switched fabric SAN's, users are generally advised to implement
FC-AL until standards have been set for switched protocol. Switch manufacturers
are releasing products that communicate FC-AL and are fabric upgradeable.
Despite the delay in producing a fully supportable set of switched fabric
standards, end users can still benefit substantially from the performance gains
within SAN architectures using FC-AL.
The potential of SAN technology is staggering. For starters, fiber optic
technology offers an unprecedented leap in bandwidth capacity. Existing SAN
backbones support 1.025 GB throughput; 2GB will be available sometime in 2000
and additional exponential leaps will occur in increasingly shorter timeframes.
As bandwidth becomes a commodity, data exchange will be unfettered and storage
will be measured in multiple petabytes (1 Petabyte=1000 Terrabytes). To meet the
demand for fiber interfaces, storage vendors are designing their platforms with
fiber backplanes, controllers and disk modules. Future offerings include "serverless"
backup technology. This feature will remove the traditional server interface
from backup libraries and enable faster backups. Presently, heterogeneous
platforms may only share the physical storage space within a SAN. As standards
emerge, Unix, NT and other open systems will enable data sharing through a
common file system.