In-Depth

The Fuss Over iSCSI

IP storage standard could benefit mainframe environments

• By Stephen Swoyer
• 03/27/2003

Research firm Gartner Inc. recently predicted that by 2006, iSCSI will be the dominant topology for connecting SANs. But while iSCSI may eventually become a force to be reckoned with in distributed environments, industry watchers aren’t convinced that it will have an immediate impact in mainframe environments.

Take independent storage consultant Arun Taneja, for example. Although optimistic about iSCSI, Taneja is concerned that the IP storage protocol simply doesn’t make for a very compelling value proposition in mainframe environments—at least not right now. “The world of mainframes has been incredibly more efficient from a storage management point of view compared to open systems. Storage is consolidated and local. I just don't think there is a strong drive to add this functionality to mainframes, at least for the foreseeable future.”

In an interview conducted last year, Mark Bakke, technical lead for the router business unit with networking giant Cisco Systems Inc., agreed. “There has been very scarce discussion in the industry of iSCSI in mainframe environments. The clear sweet spot for iSCSI, at least initially, is to provide low to mid-tier servers [NT, Win 2K, etc.] both locally and remotely access to Fibre Channel SANs,” he notes.

Speaking in an interview last year, Jim Tuckwell, marketing manager for enterprise storage, IBM Systems Group, suggested that because iSCSI was so new, it probably shouldn’t be considered as a solution for high-performance mainframe environments until the technology itself matures—and certainly not before iSCSI devices are capable of providing performance similar to ESCON and FICON. “zSeries environments tend to be some of the most demanding and represent the most sophisticated, high performance, high scale, high availabilityenvironments. iSCSI is a maturing and emerging technology.”

If iSCSI is to eventually penetrate mainframe environments, most observers say, it’ll probably do so, initially at least, in the context of Big Blue’s zSeries or S/390 mainframes running instances of Linux in z/VM or on IFLs.

“One potential scenario for … mainframe environments may be in the context of providing pooled I/O to support Linux in Logical Partition," IBM’s Tuckwell suggested. “As iSCSI takes off in the marketplace and in the context of server consolidation on Linux/zSeries, this may change the picture and create demand for iSCSI in the mainframe space too.”

Of course, before iSCSI is to be adopted in the mainframe space at all, it’s going to have to be supported—either natively or through emulation—as a channel interface on S/390 or zSeries mainframes. But Taneja and other analysts say they’re not aware of any vendor that has disclosed plans to design an iSCSI channel interface for S/390 or zSeries mainframe systems.

Unfortunately—or fortunately, depending on your perspective—neither IBM’s S/390 nor its zSeries mainframe systems offer support for the venerable Peripheral Component Interface (PCI) standard, which in the open systems space is supported on everything from low-end desktop machines to high-end Unix servers. Rather, access to storage in S/390 or zSeries environments is most often facilitated by means of IBM’s Enterprise Systems CONnection (ESCON) or Fibre CONnector (FICON) channel interfaces, both of which are proprietary in nature, and neither of which supports a standard SCSI command set.

“ESCON and FICON carry a command set call CKD [count-key-data] for disks, which offloads part of the file system or database structure to the disk controller itself,” explained Cisco’s Bakke. “Most mainframe storage is dependent on this structure, so ESCON and FICON are not easily replaced by Parallel SCSI, Fibre Channel, iSCSI, or any other logical-block-based command set.”

What all of this means, says Taneja, is that any vendor that wants to design an iSCSI channel interface for IBM mainframe environments has its work cut out for it. While it’s possible to do iSCSI emulation on either the ESCON or FICON channel interfaces, analysts stress that—in transaction processing environments, especially—performance will suffer.

“If higher performance is needed—[which is] usually the case with mainframes—an iSCSI NIC might be needed. If the mainframe supports PCI, this would only require a driver to be written; if not, someone would have to build a channel interface capable of offloading iSCSI,” Cisco’s Bakke concluded.

Nevertheless, there’s some reason to be optimistic. First of all, notes Taneja, S/390 and zSeries mainframes outfitted with a FICON channel interface can participate in storage area networks (SAN) and share a common storage pool with heterogeneous open systems. In addition to Big Blue, storage giant EMC Corp., ships one such SAN solution—Symmetrix—for IBM mainframe systems. It’s conceivable, then, that vendors will incorporate iSCSI support into SAN products such as these, he speculates.

Notwithstanding its low cost, iSCSI offers other compelling advantages in mainframe environments. First, because of its IP-based underpinnings, it’s not constrained by the distance limitations of fibre channel (10 KM) or even of IBM’s comparatively robust ESCON and FICON channel interfaces (approximately 40-60 KM). As a result, it’s ideal for backup, disaster recovery or replication between or among several geographically dispersed sites.

Said Jim O’Conner, director of marketing with Bus-Tech Inc., a Massachusetts-based storage appliance vendor, in an interview last year: “I look at iSCSI and it’s kind of like a free channel extension. It’s ideal for remote data replication and data vaulting. It just makes it so simple."