Published: 12 Jul 2006
TEN GIGABIT ETHERNET (GbE) is just around the corner, and a host of storage applications are poised to take advantage of it now that Microsoft has announced native support for TCP offload hardware as part of its Scalable Networking Pack (SNP) announcement in May.
SNP is an architectural enhancement for Windows Server 2003 that's available as a free download and will also ship as an integral part of Windows Vista and Longhorn. SNP adds three main features to Windows: TCP "Chimney" offload, which allows the operating system to offload TCP processing onto specialized hardware away from the main CPU; receive-side scaling, for better utilization of dual-core processors; and NetDMA, or direct-memory access.
The most obvious beneficiary of SNP is iSCSI, especially at 10Gb/sec speeds. At 1Gb/sec speeds, most iSCSI SAN practitioners have found they can get adequate performance from a software iSCSI initiator. "At 1Gb/sec, TOE [TCP offload engine] doesn't matter," says Marc Staimer, founder and senior analyst at Dragon Slayer Consulting, Beaverton, OR. "Today's CPUs are powerful enough."
But as Ethernet moves to 10Gb/sec, doing iSCSI without a TOE may not be feasible. According to Ian Hameroff, Microsoft's senior product manager for Windows Server, a good rule of thumb is that you need 1GHz of processing power to drive every 1Gb/sec of throughput. In other words, without TOE, you'd need 10GHz of CPU power to drive a 10Gb/sec pipe.
Even if the raw CPU power is there to drive network traffic without TOE, the CPU utilization might be so high that it doesn't leave room to run the main application, explains Allen Light, Broadcom Corp.'s product line manager for the NetXtreme II line of network interface cards that support SNP.
Enter iSCSI host bus adapters (HBAs), like those offered by Alacritech, Broadcom and Chelsio Communications. Beyond fighting to prove their worth as performance enhancers, they've also suffered from Microsoft compatibility issues. "In the Windows world, there are two ways to do things," says Joe Gervais, Alacritech's director of product management. "You can make something work, but in a way that isn't sanctioned by Microsoft. Or there's the Microsoft-sanctioned way." Pre-SNP, "we would have to set aside the Microsoft TCP stack," he says, but with SNP in place, its TOE hardware works directly with the native stack.
iSCSI isn't the only thing that SNP can help. Generally speaking, "any long-lived, large-sized data traffic can benefit from SNP," says Microsoft's Hameroff, especially file serving, but also backup and replication. In fact, given the way TCP Chimney works, some believe SNP will do more to improve overall network speed than to further iSCSI. Several server vendors, including Dell, Fujitsu, Hewlett-Packard and NEC, have stated they'll deliver SNP-compliant servers. Announcements are expected as soon as this month.
TCP Chimney, explains Dragon Slayer's Staimer, uses a split-stack approach in which the operating system passes along the vast majority of TCP traffic to the specialized hardware, but handles exceptions within the main CPU. But because the iSCSI protocol terminates the TCP traffic, "full TCP offload makes more sense," he says. The split-stack approach, however, lets silicon providers to keep their TOE hardware relatively simple and robust. "It narrows the scope of what needs to be off-loaded," says Broadcom's Light.
Another question is whether specialized iSCSI HBAs will live on or whether they'll be supplanted by generic network-acceleration hardware, either on a separate card or directly on the motherboard. Alacritech sells both, and Gervais says that with SNP finally here "we think customers are going to prefer the universal network accelerator rather than buying two different parts--one for the LAN and one for the SAN."