In-Depth

The Wide Area E-SAN: The Ultimate Business Continuity Insurance

• 11/01/1999

There is a common misconception taking hold in the technology world these days that unless an enterprise has implemented fibre channel, it cannot derive the benefits of storage networking or a storage area network (SAN). The reality is that most networks can support some type of SAN; all that’s necessary is a fabric – any network fabric – that links multiple hosts to multiple storage devices. Of course, to achieve the most significant and cost-effective business continuity insurance possible with a SAN, the fabric should support connectivity between hosts and storage devices over the wide area – but there are a variety of SAN types that can provide some degree of protection over more limited distances.

With the recognition that the purpose of a SAN is to deliver a function, rather than meet an artificial connectivity criteria, companies should make every effort to leverage their existing resources to create a SAN. Failing to do so would be analogous to not implementing a client/server network with Windows NT boxes, because many individuals say that to be a server, a device must be UNIX-based. It does not matter what types of hosts (servers or mainframes) or storage devices (JBOD, RAID or automated tape libraries) are associated with the SAN.

In short, a SAN represents a conceptual framework that can encompass all types of hosts and storage devices, as well as differing communications channels, including conventional LANs, fibre channel, bus and tag, ESCON and SCSI. As a result, there are a number of SAN types, each providing varying levels of functionality. However, to achieve the benefits of an effective enterprise storage networking solution or enterprise SAN (E-SAN), wide area connectivity is required, and the only way to achieve this, at the present time, is with channel extension.

SAN is not the only form of storage networking available today. Another popular form of storage networking is accomplished with network attached storage (NAS) devices. By placing intelligent storage devices, or "filers," on an existing communications network backbone, storage resources can be shared among multiple servers. While this can be a cost-effective approach, the bandwidth limitations of the communications network can severely limit the type of storage data – they cannot support transactional traffic, being restricted to files only. Furthermore, tape libraries attached to a NAS are much slower than they would be on a dedicated channel-based network, which has been optimized for bulk data transport.

For improved storage networking performance, many enterprises turn to fibre channel technology (FC) and an FC-SAN. With the ability to carry any kind of data traffic, the FC-SAN offers at least three times the performance of a NAS solution, in addition to easier management and higher transmission integrity. With an FC-SAN, data is never lost to congestion, as it may be with a NAS solution. However, on the downside, FC-SANs are only functional to a maximum distance of 30 kilometers. By comparison, a NAS solution can span the globe because the typical enterprise communication networks rely on standard router technology and IP protocols.

Another type of SAN, based on mainframes, is the Sysplex SAN (S-SAN). Relying on either bus and tag technology or ESCON, S-SANs were really the first type of SAN. Developed back in the early 1960s with bus and tag technology, the original S-SAN suffered the limitation of only supporting a limited number of devices within 400 feet – even with the addition of a matrix switch. Furthermore, performance was limited to 4.5 Mbps.

ESCON picked up where bus and tag left off, offering an S-SAN with a throughput of 17MBps, the ability to link to a greater number of devices and a distance of 20 kilometers (for multimode fiber).

For faster performance where FC is not available, enterprises may also consider a SCSI-SAN. Modeled on bus and tag architecture, wherein devices are daisy-chained over parallel channels, the latest version of SCSI-SANs can support speeds of up to 80 Mbps, approaching the 100 Mbps of FC-SANs.

While these various types of SANs all meet a localized storage need, none can, in and of themselves, provide support over long distances. This is a severe shortcoming as companies increasingly want to leverage distributed resources to achieve increased utilization of storage resources, and common management of storage, over a WAN (wide area network). Even more important, only by linking widely dispersed processing and storage resources can business continuity be ensured in the event of a local disaster. As a result, companies are now looking for ways to achieve the distance capabilities of NAS with all the functionality of an S-SAN – a high performance enterprise storage networking solution.

The good news is that this goal can be met with a time-proven technology – channel extension. What channel extension offers is the only way to currently share data storage across distributed geographic locations by leveraging existing SANs. The need for this high level of connectivity cannot be understated in light of the increasing importance of data to the success of virtually every enterprise. Unless business continuity is ensured with remote data storage and processing capabilities, businesses, quite simply, can fail.

Consider that industry analysts estimate that every enterprise will suffer a major disaster at least once in every 20-year period. While that interval may seem very distant, what happens if the disaster occurs tomorrow? Remember: In a world where companies rely on 24x7 operations, the cost of downtime can be enormous. Downtime at an ATM, for example, can cost a bank $14,500 per hour, but at an airline reservations center, downtime can cost $90,000 per hour. And, at a brokerage firm, the downtime cost soars to $6.45 million per hour.

The only solution to avoiding these downtime costs is by providing an environment that supports remote data mirroring or hot backups with electronic tape vaulting over the wide area. Either of these approaches ensures that storage and processing capabilities at remote facilities are utilized to prevent a corporate failure from occurring, should a disaster hit a single, local site.

Although some storage management product vendors say that an alternative solution is a distributed backup system that places a backup server in front of a tape library, this approach, being file system-based, does not protect transactional data. Furthermore, because these systems ultimately rely on the LAN, they are subject to bandwidth and latency issues that can cause collisions, a reduction in data integrity and possible service interruptions for other LAN users. By comparison, electronic tape vaulting solutions that are based on E-SANs and rely on channel extension, can encompass all types of data, complete backups in real time and do not affect bandwidth utilization on the LAN. As a result, only electronic tape vaulting can provide an alternative for data mirroring, but one that may not be appropriate for online transaction processing environments.

For maximum protection across a WAN, in situations where downtime must be absolutely minimized, there simply is no better solution than data mirroring, also referred to as remote copy. With all production data duplicated across an E-SAN, if one site suffers a disaster or outage – planned or otherwise – of any kind, at any time, operations can be by-passed from this site to another in the WAN, with little or no business interruptions.

But, in order for this objective to be fully met, it is essential that the channel extension-based SAN minimize latency. The reason: Every millisecond of latency that occurs in the channel extender is the equivalent of 125 circuit miles. So, the more distance lost through latency, the fewer real circuit miles available for the E-SAN.

A low-latency channel extender also significantly minimizes the impact of remote mirroring on product application response time. When data is mirrored in real-time (known as synchronous mode), applications must wait until their data is mirrored before they can complete a processing command. Suppose data from an ATM is being mirrored remotely from one location to another. If a customer enters a command, the ATM would not be able to finish processing the transaction until the data involved had been mirrored from one site to the other. A channel extender with standard latency will compound this delay by adding significant time to move data through its system to the wide area network.

In contrast, a low-latency channel extender will move data very quickly, which minimizes impact on response time. A low-latency channel extender also allows the customer to send data farther without increasing response time. When data is sent farther, effectively, the time savings from the channel extender is used to travel additional miles across the wide area network instead. In short, a low-latency channel extender will send data faster and farther, while minimizing the impact to application performance and responsiveness.

Another important issue in optimizing performance of a wide area E-SAN is data compression. Data compression can help to significantly reduce wide area network costs by minimizing the bandwidth required for the application. Channel extension solutions that provide data compression on the channel side of the extender (channel compression) provide significantly better performance and bandwidth utilization than channel extension solutions that provide data compression on the network link side of the extender (link compression).

Not to be overlooked is the ability of the channel extenders to support all mirroring protocols provided by RAID vendors, including SRDF, XRC, PPRC, HXRC, HRC, APRC, etc. – without software changes. All industry standard automated tape library devices must be supported as well.

Of equal concern should be the ability of the channel extender to interface not just with an S-SAN, but with other SAN types in the enterprise, as well. Only then can a true E-SAN be created that protects the entire enterprise and leverages corporate resources, enterprisewide.

One final note: To streamline management over the increasingly complex wide area E-SAN, and all SANs it encompasses, be sure the channel extension solution supports SNMP. This capability will dramatically streamline management of the entire E-SAN – including the channel extenders – with existing network management systems.

About the Author: Dave Hubbard is Vice President of Marketing at Computerm Corportion (Pittsburgh), a worldwide provider of data communication systems. He has more than 25 years of experience in the industry.