The Emergence of 4Gb SANs

For the past six months, there has been extensive discussion throughout the storage industry as to when 4Gb fibre channel SANs would emerge. As 2005 begins, storage vendors are preparing to bring to the market 4Gb Fibre channel technology that is twice as fast as previous generations of Fibre channel.

Is now the time to consider an upgrade?

In order to determine the advantages of 4Gb fibre channel technology we should examine the types of applications that utilize the SAN.

Many organizations adopted SAN solutions to take advantage of the performance they offer. There are two ways to measure performance of a SAN device: mega bytes per second (MB/s) and inputs/outputs per second (IOPS). Initial 4Gb SAN products have very impressive performance numbers.

For example, the Brocade 4100 SAN switch offers aggregate throughput of 256Gb/sec with trunking up to 32 Gb/sec and SGI’s initial 4Gb storage array, the SGI TP9700, offers 1,600 MB/sec throughput with up to 475,000 IOPS.

These performance numbers will only increase as products mature. So what do these impressive performance numbers mean to the enterprise? A 2Gb storage array requires twice as many host ports as a 4Gb array to deliver the throughput of four 4Gb ports. This frees four ports in the SAN for each array.

On the host side, if a server requires more than two 2Gb HBAs (host-bus adaptor) to attain the required performance (a server should have at least two HBAs for availability), then with 4Gb that number can be halved.

Fewer HBA’s means fewer switch ports used and less overall cost.


Another common reason for deploying a SAN is to implement a robust backup solution.

With the introduction of large-capacity SATA disks, SAN-based disk-to-disk-to-tape solutions have become common. Companies have realized dramatic reductions in backup and recovery times.

Since most vendors that are releasing 4Gb SAN products are also upgrading the products to take advantage of the performance that is now available, replacing a 1Gb or 2Gb SATA device with a 4Gb device can further enhance the solution.

Many times, disk-to-disk backup solutions have a bottleneck in the disk attached to the backup server(s). To eliminate the bottleneck, more storage arrays are needed to spread the load.

Using a 4Gb storage array will allow for fewer storage devices behind the backup servers. Since the 4Gb storage device will be able to handle much more load, more backup servers can also be deployed, increasing overall backup performance.


In the past, replication solutions were far too expensive for most companies to implement. Today, as prices have dropped, more and more companies are able to deploy data replication technologies.

Replication that occurs on a local SAN (i.e. not over a WAN) will see obvious improvements in performance if the SAN infrastructure is upgraded to 4Gb.

Even if only the destination storage device is 4Gb the delay inherent in synchronous replication will be less. Also, if multiple storage arrays are replicating to one destination storage device, upgrading that storage array will allow more incoming replication streams without further impacting performance.

One of the key advantages of 4Gb technology is that it is backward compatible with 2Gb and even 1Gb. That means that you need not replace your entire SAN with 4Gb technology, but can add the new technology incrementally. Of course, 4Gb products will slow down to the 2Gb or 1Gb speed if they are connected, but zoning can allow a rolling upgrade strategy with minimal disruption.

However, there are three possible disadvantages to adopting 4Gb SAN technology: maturity, availability and cost.


There is risk involved whenever adopting a new technology. The risk, in this case, should be minimal.

History shows that the move from SCSI to fibre channel was, at times, challenging. Initial Fiber Channel implementations were immature technology, with some problems. As fibre channel products matured, they became much more stable.

The move from 1Gb fibre channel to 2Gb fibre channel was practically bug free. Most issues were minor and easily solved. Vendors have learned from experience during the 1GB to 2Gb transition. Expect the move from 2Gb to 4Gb to be even smoother.


Today, 4Gb switches and 4Gb storage devices are starting to appear. HBAs are just around the corner. By the end of Q2 2005, 4Gb SAN technology will be available from most of the major storage vendors.


Today, 4Gb SAN components are very close in price to 2Gb SAN components and well worth any small premium.

As 4Gb technology grows in availability, the cost of 2Gb SAN components will drop as did the cost of 1Gb SAN products when 2Gb appeared. But be wary of saving a few dollars when buying 2Gb SAN components past Q2 2005, since 1Gb SAN devices disappeared pretty quickly after 2Gb came on the scene.

Investing in 2Gb SAN devices when a 4Gb option is available is almost certain to cause support issues in the not too distant future.

Should all existing SAN gear be immediately scraped for the new 4Gb technology? Of course not. Should new SAN equipment purchases be 4Gb technology? Absolutely!

Resolving I/O bottlenecks is typically the most complicated and time consuming problem faced by a systems administrator. It is refreshing to see how SAN technology keeps improving rapidly and stably to address this area and to keep bringing more efficiencies to the enterprise.

Jim McKinstry is senior systems engineer with Engenio Information Technologies, an OEM of storage solutions for IBM, Teradata, Sun and others.