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One of the most promising areas of storage technology today

is SAN-to-SAN connectivity.Extending storage area net-

works (SANs) beyond the local fabric will greatly improve

data availability, disaster avoidance,and storage consolidation.Long-

haul SAN transportthe ability to connect geographically disparate

SANs and make them look like one fabricprovides a simplified

management model and allows consolidation of key human resources.

Long-distance SAN-to-SAN connectivity is also important for

storage service providers (SSPs) because it allows economic and

business needs, rather than location, to dictate the placement of

consolidated storage facilities.

Unfortunately, confusion exists in the marketplace regarding

SAN-to-SAN connectivity. Multiple protocols (some based on open

standards and some proprietary), competing technology camps, and

a range of opinions on what is considered the best option for SAN

connectivity are sending mixed signals to the IT community. This

uncertainty threatens to delay deployment of this useful technology

to the detriment of business operations. However, a proven technol-

ogy, which has been in existence for years, is mirroring from one

SAN to another. Adding a relatively new technologysnapshot and

virtualizationto mirroring can create a powerful combination

that many IT organizations can implement today. (See Figure 1.)

Current approaches to SAN connectivitySeveral distinct methodologies enable SAN-to-SAN connectivity.

The most widely deployed method of extending SANs past local

fabrics is either server or SAN appliance based. Integrated solutions

based on existing local area network (LAN) and wide area network

(WAN) infrastructures, Fibre Channel over Asynchronous Transfer

Mode (ATM) and Synchronous Optical Network (SONET), and

Fibre Channel-to-T1/T3 are quickly emerging. The viability of

each methodology depends on organizational data availability needs

and the requirements of different storage applications. The most

common of these applications includes storage consolidation,

disaster avoidance/recovery, backup, and virus protection.

Server-based

Server-based approaches employ a server with host bus adapters,

network interface cards (NICs), and specialized software for routing

either Fibre Channel packets or Small Computer System Interface

(SCSI) blocks to a LAN for transport over an existing WAN. Server-

based solutions can be deployed immediately but often complicate

long-distance SAN transport.

Server-based SAN transport can be costly and difficult to

manage because it typically requires a high-end server with vast

amounts of memory, powerful processors, and specialized software.

These powerful servers usually support mission-critical applications

and cannot be dedicated only to data transportation.

Appliance-based

Appliance-based approaches employ a SAN appliance specifically

designed and architected for the virtualization and movement of

Multiservice Accessfor SAN-to-SANConnectivity

SAN-to-SAN connectivity promises interoperability among geographically dispersed storage

area networks. This article describes several methods for SAN-to-SAN connectivity and outlines

the benefits of a multiservice access approach.

By Matthew Brisse and Michael Harris

www.dell.com/powersolutions PowerSolutions

S T O R A G E E N V I R O N M E N T

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data within a SAN and Internet Protocol (IP) environment. SAN

appliances include specialized hardware and software to provide

logical unit number (LUN) masking and mapping techniques while

combining high-end features such as LUN virtualization, LUN con-

catenation, sub-LUN partitioning, snapshot, and mirroring func-

tions. Advanced appliances such as the Dell PowerVault 530F can

perform synchronous Fibre Channel mirroring as well as synchro-

nous and asynchronous IP-based mirroring.

SAN appliances combine virtualization techniques with snap-

shot and mirroring technologies. This combination enables admin-

istrators to create point-in-time clones of LUNs and then mirror the

LUN to a remote site where the targeted LUN can be assigned to a

server for backup or testing, or staged in the event of a disaster or

virus attack. The mirroring transport for appliances is typically

Gigabit Ethernet or Gigabit Ethernet over ATM. For online trans-

action processing (OLTP) and data-sensitive applications, adminis-

trators should profile the latency requirement to ensure adequate

data response. Gigabit Ethernet and SONET (OC-12 or higher) are

adequate transports for most applications, depending on the dis-

tance that the data will travel.

Gigabit Ethernet

Another approach is to deploy devices that leverage current Ethernet-

based network infrastructures to bridge Fibre Channel SANs and

existing LAN-to-WAN routers and switches. Do not confuse this

method with storage over IP systems in which all storage traffic,

including SCSI block I/O, is run over an Ethernet or Gigabit

Ethernet network.

The obvious advantage of transporting Fibre Channel packets

through existing LANs and WANs is that most of the infrastructure

is already in place. In addition, the IT community is very familiar

with its current infrastructure, resulting in faster implementation

and easier management.

The major disadvantage is congestion and latency. Running block

I/O traffic with other traffic can overload the primary network and,

during periods of heavy usage,prevent storage traffic from arriving in

order and on time. Data must be routed through multiple switches

and routers before it enters the WAN, which compounds latency

problems and complexity. For these reasons, the Fibre Channel-to-

Gigabit Ethernet approach best fits the following scenarios: organiza-

tions that have existing infrastructures, emergency failover from a

dedicated storage WAN link, periods when network traffic is light

(for example, after business hours), and situations that require low-

to moderate-speed bandwidths for financial considerations.

SONET

In many cases, a dedicated connection to remote storage resources is

preferred, especially when this connection is constantly available or

when large amounts of block I/O traffic are expected to flow

between geographically dispersed SANs.

S T O R A G E E N V I R O N M E N T

PowerSolutions Issue 4, 20012

IPnetwork

Fibre Channelswitch

Fibre ChannelDirector

WAN

OC-48

SAN

PSTN Remotemanagement

Alarm/management call

T3

Ethernet

SNMPSAN management

DRbackup

SAN

OC-3

Router

GigabitEthernet

ASPSAN

SSPSAN

ATM

Servers

Disk arrays

EnterpriseSAN

......

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Figure 1. Protocol-neutral multiservice access allows flexible connectivity options

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SONET (OC-1 and up) provides effective SAN-to-SAN connec-

tivity because of the high throughput (up to 2.5 Gbps), multipoint

service, and more reliable error handling. Disadvantages of SANtransport via SONET include its relatively short range (restricted to

metropolitan area networks), high cost, and lack of availability to

many enterprises. A typical list price for SONET (OC-12 to OC-48)

in the New York City area can be as high as $300,000 per month.

SONET (OC-48) is a good choice when running I/O-intensive

applications over short distances. However, the storage application

must be mission-critical to justify the expense of dedicated solutions.

Network latency considerationsThe issue of network latency is fundamental to the intrinsic

nature of data networks. Distance, packet loss, network conges-

tion, link errors, and network errors cause latency. Storage appli-cations such as databases and OLTP are extremely sensitive to the

effects of data latency. Even in a perfectly engineered network, link

errors occur. For example, consider the Fibre Channel objective of

10-12 bit error rate. For a 10 Gbps link, this metric equates to one

error every 100 seconds.

Network errors occur with significant frequency in IP networks.

Transmission Control Protocol (TCP) catches network errors that

occur at a ratio between 1 packet in 1,100 and 1 in 32,000. Cyclical

redundancy check (CRC) does not catch these errors. Typical IP

network characteristics that permit 3 percent to 5 percent packet

loss are potentially devastating not only to data latency, but also to

the overall throughput of the application solution.

The amount of possible errors in a typical data network can

be exponential when compared to that of a Fibre Channel network.

In a direct-connect (server to storage) scenario, the average latency

is 3 milliseconds to 5 milliseconds with most of the latency associ-

ated with the disk drive mechanics. Additional latency from errors

or distance associated with IP networks might affect the storage

applications themselves. Storage applications such as OLTP have

very stringent latency requirements. Proper throughput sizing is

imperative when considering SAN-to-SAN connectivity.

Open vs. proprietary protocolsTransport protocols are another important element of SAN-to-SAN

connectivity. Open standards-based systems compete with propri-

etary protocols.

Many vendors are backing the open protocol standards devel-

oped by the major standards bodies. These protocols include

Fibre Channel (FC) over Internet Protocol (IP), which is cur-

rently under discussion by the Internet Engineering Task Force,

and Fibre Channel Back Bone, which is under revision by the

American National Standards Institute T11.3 committee. Both

proposed standards represent an attempt to develop an open

protocol for transporting Fibre Channel SANs over IP, ATM, or

SONET networks.

Protocols specific to a particular vendors product also exist.This approach works well if a customer is solving a specific need

and is an early adopter of technology. However, proprietary systems

can lock IT managers into a specific vendors solutions, which may

not serve all of their future needs effectively. Proprietary systems

offer little protection against technology obsolescence.

Multiservice access can support different protocolsFaced with so many compelling and competing approaches, ven-

dors, integrators, and end users are challenged by questions such as:

Which type of connection should I use? Which protocol is the right

one? Whose approach will dominate in the end?

Many IT professionals with responsibility for multiple storageapplications believe they have no choice but to deploy different ven-

dors products, many of which are not interoperable. This decision

is a risky career move unless purchased from a tier-one player that

has tested, certified, and promised to stand by the entire solution.

An alternative to the multivendor solution is the multiservice

access approach. Multiservice access provides a variety of WAN,

LAN, and metropolitan area network (MAN) interlinks that

support different protocols. Such access offers IT managers the

opportunity to make decisions based on need and budget. Multi-

service access frees IT professionals from being at the mercy of a

particular vendor. The applications, rather than the router or

switch vendor, can now drive the connectivity planning. By

deploying a variety of open protocols, IT managers can inter-

operate effectively and efficiently with other facilities, customers,

and suppliers. (See Figure 2.)

The multiservice access concept has been a fundamental ele-

ment of traditional networking. When applied to SAN-to-SAN

connectivity, multiservice access provides IT managers with the

right tools for meeting the constantly escalating storage demands

of their enterprise environments.

Multiservice access offers five advantages:

Maximized performance. The needs of mission-critical

applications can determine WAN-link deployment

decisions. Multiservice access also can maximize

application performance.

Tighter cost control. Bandwidth can be purchased based

on need, which supports tighter cost control. WAN links are

a recurring monthly expense.

Investment protection. IT managers can order incremental

upgrades in connectivity with minimal changes and disrup-

tions to the overall system, ensuring investment protection.

As the system requirements grow, decisions can be based

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on economics rather than a particular vendors product

schedule.

Efficient service. Various groups within the enterprise can

use a solution best suited to their scenario; for example,

SONET (OC-48) is not universally available. Even within a

single company, WAN infrastructure varies widely. Accord-

ingly, relying on one approach for SAN-to-SAN connectivity

means that some locations cannot use an application until

that service is available. If an organization implements a

slower interconnect such as a T1, some bandwidth-intensive

applications will either run at an unacceptable slower

speed or, even worse, fail because of unexpected latencies

that were not anticipated by the application designer.

Enhanced interoperability. As SAN-to-SAN connectivity

becomes increasingly important, the interoperability of

equipment from multiple vendors and different connectivity

options is vital. Moreover, in a time when mergers and

acquisitions are commonplace, interoperability is an impor-

tant part of IT planning and should be considered when

deploying SAN-to-SAN applications.

Vendors have announced multiservice access devices: the

Cisco SN 5420 Storage Router, the Nishan Systems IPS 3000

SoIP Storage Switch, and the Entrada Networks Silverline-222

SAN-over-IP transport switch. The Silverline-222, for example, can

connect SAN islands across a variety of network topologies. Con-nection options range from T3 for ATM-based WANs, OC-3 and

higher feeds for WAN/MAN networks, and Gigabit Ethernet for

transporting SANs over existing high-speed IP networks. With

products like the Silverline-222 and others on the horizon, the out-

look is promising for IT professionals faced with the daunting task

of connecting SAN islands in todays environments.

Multiservice access offers viable solutionThe SAN-to-SAN connectivity landscape is new and fragmented.

Several different schools of thought have emerged regarding the

correct way to allow geographically dispersed SANs to inter-

operate. Advancements in Fibre Channel, Gigabit Ethernet, WANinfrastructure, and other key storage networking technologies are

making the multiservice access approach a reality, and one that

customers should consider.

Matthew Brisse (Matthew_Brisse@Dell.com) is the SAN product

manager for PowerVault 530F, Dells SAN virtualization appliance.

He has worked in the storage industry for more than 19 years in

engineering development and product management roles. He holds

a B.S. in Sociology, Statistics and Analytical Research from the

University of Wisconsin.

Michael Harris (MAHarris@entradanet.com) is senior vice presi-

dent of Entrada Networks, where he is responsible for marketing,

business development, and investor relations. Michael holds an MBA

from Vanderbilt University and a B.S. in Communications from the

University of Tennessee.

S T O R A G E E N V I R O N M E N T

PowerSolutions Issue 4, 20014

FOR MORE INFORMATION

Cisco Systems: www.cisco.com

Dell Computer Corporation: www.dell.comEntrada Networks: www.entradanet.com

Nishan Systems: www.nishansystems.com

American National Standards Institute: www.ansi.org

Internet Engineering Task Force: www.ietf.org

SAN SAN

Storagedisk

mirroring

SAN

Long haul

Local

Leased line,Dark Fiber

DWDM, ATM,and/or IP

Site A Site B

Site C

Disasterrecovery

Remotebackup

Figure 2. Multiservice SAN transport supports many networking applications