Explainer: The skinny on iSCSI SANs

In what was a coming-out party for the Internet Small Computer System Interface protocol, these debuting products tested by the iLabs IP storage team showed a high level of interoperability and a surprising level of functionality.

The charter for the iLabs event had three planks:

-- To demonstrate multivendor interoperability over an Ethernet infrastructure.

-- To connect remote Fibre Channel storage-area networks (SAN) via iSCSI links.

-- To demonstrate how iSCSI can be used over an IP enterprise backbone or WAN for business continuance measures such as remote data replication.

Though the upshot was the surprising degree to which vendors were able to accomplish these goals, the themes that developed were perhaps even more compelling.

Inside iSCSI

ISCSI adapters essentially are PCI-based network interface cards and SCSI controller hybrids. They are installed into Unix or Windows NT/2000 servers to make them initiator (vs. target) clients on an iSCSI network.

Consensus among the iLabs engineers was that the bulk of the iSCSI issues requiring attention stand on the adapter side of the SANs equation. The encumbrances of TCP/IP offload, lack of driver standards and no explicit Microsoft Corp. support for the iSCSI protocol have been negotiated by different vendors in different ways.

Recent criticism of iSCSI adapters focuses on their TCP/IP offloading functions. iSCSI's TCP-based architecture creates a huge processing overhead on the host system's CPU. To address this, most iSCSI adapters have a separate TCP/IP stack in the hardware running parallel and invisibly to the stack supporting the host operating system. Dual, independent TCP/IP stacks are a problematic proposition -- no shared routing or Address Resolution Protocol tables, no load balancing or failover, and limited visibility of the iSCSI adapter from the operating system are some of the fundamental concerns.

Alacritech offered a unique approach to this problem. Alacritech Inc.'s 1000x1 Server and Storage Accelerator does not offer full TCP/IP offload from the host operating system. Via its device driver, the adapter only performs data movement, while control and error handling remain the duties of the host's native IP stack.

While all the other iSCSI adapters in the iLabs testing appeared as SCSI controllers to their Win 2000 host servers, Alacritech's technology was visible only as a NIC. This enabled the use of traditional IP connectivity tools such as ping and traceroute through the iSCSI NIC.

ISCSI adapters provided by Adaptec Inc., Alacritech, Emulex Corp., Intel Corp. and QLogic Corp. all passed muster at the prestage, interoperating easily with the other iSCSI network elements without incident. They came preinstalled on off-the-shelf Win 2000 servers, and although some required more work than others, volunteers said the adapters were among the more painless network elements to deploy.

While iSCSI adapter vendors and gateways were well represented, QLogic was the only major Fibre Channel switch vendor that supported the event, sending its SANbox2 Fibre Channel switch. IBM Corp., a major iSCSI proponent, provided a TotalStorage IP Storage 200i as an iSCSI target. Spectra Logic Corp. sent an iSCSI-based Spectra 2000 Tape Library. Among Fibre Channel-based targets, only Dot Hill Systems Corp. was present with a SANnet 3300 RAID array to go with an Axis Storage Manager server.

Vendor support for the event also included Lightpointe, which provided FlightPath transceivers for a wireless Gigabit Ethernet link. FalconStor Software Inc. provided multiprotocol SAN management software called IPStor. Finisar Corp. came with a variety of test gear, and the Layer 2/3 infrastructure was anchored by Cisco Systems Inc. and Foundry Networks Inc. gear.

Fibre Channel meets iSCSI

The most notable technical innovation on the floor was the ability of iSCSI gateways to seamlessly expose iSCSI and Fibre Channel storage network elements to each other. This proves that iSCSI can be deployed not only as an end-to-end storage networking protocol, but it also can provide a migration path for existing Fibre Channel gear.

Two prominent iSCSI flag bearers, Cisco and Nishan Systems Inc., contributed iSCSI gateways. The Cisco SN 5420 Storage Router has two fixed dual-SC interfaces, one designated for Gigabit Ethernet, the other for Fibre Channel. It demonstrated the ability to expose iSCSI initiators with Fibre Channel storage arrays and "just a bunch of disks" across an IP network.

Like the SN 5420, Nishan's IPS 3300 can expose Fibre Channel target storage devices to servers fitted with iSCSI adapters. What's different, however, is that Nishan claims full support for the opposite scenario -- Fibre Channel host servers interoperating with iSCSI-based storage arrays.iSCSI for business continuance?

With all the rhetoric being thrown around about iSCSI as an alternative to Fibre Channel, some fundamental differences are getting lost. ISCSI has a home in an optimal business-continuance model only where it coexists with Fibre Channel networks.

"ISCSI is not architected for replication and disaster recovery. It's for database applications, e-mail and other online, transactional processes," said Mike Frase, technical support engineer for Cisco and iLabs Storage Team leader. "For moving huge gobs of data quickly, Fibre Channel over an IP link is probably the way to go."

The real case for iSCSI, Frase said, is in the numbers. "Find a Fibre Channel network that is at even 50 percent of capacity. ISCSI allows you to do other things besides storage with your infrastructure, which gives better ROI on your infrastructure dollar," he said.

Percy is a technology analyst at Miercom, an independent testing lab in Princeton Junction, NJ. He can be reached at kpercy@mier.com.

Testing goal: Explore wireless security options and demonstrate 802.1x interoperability.

Products tested: 40

Fact found: Choosing an authentication method is a defining decision in terms of which 802.1x-based products you can use together in your wireless network.

Sport most resembling technology: Horseshoes - where "close" is almost as good as "touching."

MPLS testing tally

Testing goal: Test interoperability for Layer 2 cross-connect and Layer 3 VPN and RSVP services.

Testing engineers: 10

Fact found: In the Layer 2 cross-connect test, 85% of the products tested were able to interoperate with the other products tested.

Sport most resembling technology: Cricket -- a lot of flinging of balls around, and no one really understands the rules.

Storage testing tally

Testing goal: To demonstrate multivendor interoperability, connect remote Fibre Channel SANs via iSCSI links and demonstrate how iSCSI can be used over an IP enterprise backbone or WAN for business continuance.

Products tested: 30

Fact found: iSCSI equipment can front end more expensive Fiber Channel SANs gear to protect your investment.

Sport most resembling technology: NASCAR, in which the gear takes in the tired SCSI payload, modifies it to make it faster, cleans the windshield (header) and sends it out to win.

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