New solutions, new problems

The techniques used to solve the problem of storage-area network isolation may create other issues for information technology managers.

Performance bottlenecks are one concern. Fibre Channel uses a system of so-called buffer credits that determine the maximum amount of data that can flow between network switches. When all buffers are full, buffer credit starvation occurs and all additional data movement ceases until more buffer credits become available, a CNT Corp. white paper reported.

Vendors have introduced flow control and buffer credit management features to address this problem. CNT's UltraNet products, for example, negotiate buffer credits with Fibre Channel switches to fulfill the maximum efficiency of the link.

Compression is another method used to improve bandwidth utilization. CNT's compression technology let the Jefferson County Public School District consolidate voice and data applications on the same IP network. Don Lohman, the Colorado school district's technical support director, said the district had considered using three T1 lines but was able to go with a single T3.

Another issue with linked SAN fabrics is fault isolation. Problems that occur on one SAN should be contained so they don't spread elsewhere. From the customer's perspective, "I want to make sure that the pain in one place is not felt at another place," said Paul Ross, director of storage networks marketing at EMC Corp.

Thomas Nosella, senior manager of technical marketing within Cisco Systems Inc.'s Storage Technology Group, said the main issue with fault isolation is to expose only those devices that need to talk to each other. Perhaps only two disk subsystems need to communicate via a wide-area network link. In that case, Cisco's Virtual SAN technology can be used to carve off a subset of ports and isolate the devices in a virtual fabric across the WAN, Nosella said.

Sanera Systems Inc., meanwhile, offers dynamic partitions as part of its DS10000 director-class SAN switch. This feature segments a switch into multiple virtual switches, or partitions.