Servers Strain Under 4-Gig FC

When Andy Tran, CTO at Pacific Title Art and Studio upgraded his 4-Gbit/s, 400-Tbyte SAN last year, he included more than switches and disk arrays. (See Pacific Title & Art Studio.) He also replaced his servers.

"I wanted to be ahead of the game," he says. "I didn't want to take the chance that the servers couldn't keep up." So on top of a multimillion-dollar installation, he shelled out for new SGI machines equipped with Itanium and Intel dual-core processors.

As a Hollywood post-production studio where Sly Stallone is a frequent visitor, PacTitle has a big budget to accommodate sizeable upgrades. But smaller firms may find costs surrounding a move to 4 Gbit/s an unpleasant surprise.

So common is the need to upgrade infrastructure around 4-Gbit/s FC that suppliers have special lingo for it. "Vendors talk about the difference between four-gig point products and end-to-end four-gig solutions," notes Charles King, principal at consultancy Pund-IT Research. The solutions, of course, include things like servers.

As Tim Arland, principal consultant for storage solutions, Forsythe Solutions Group Inc., points out in the latest Byte and Switch "Ask the Expert" feature, many server backplanes can't keep up with a 4-Gbit/s FC HBA. (See How Do I Move to 4 Gbit/s?)To avoid disappointment, experts say a server must start with the right PCI (peripheral component interconnect) bus linking HBAs to the server's motherboard. In the past, PCI bus speeds could be as low as 33MHz, which in 32-bit mode meant throughput of about 132 Mbytes/second. Many were limited to 66MHz, which resulted in throughput of about 532 Mbytes/second in 64-bit mode. With the introduction of PCI-X and rates of 133 MHz, that throughput was doubled to about 1.06 Gbytes/second. While that is sufficient to keep pace with 4-Gbit/s networks, experts suggest that to accommodate for than one FC port on the same server it's best to use PCI-X 2.0 or PCIe buses.

But there's more than the PCI bus to check. Another potential server bottleneck is the memory controller for a server's CPU. Sometimes referred to as the front side bus (FSB), this element should be capable of running at a minimum of 400 MHz, one expert says, in order to accommodate simple I/O related to 4-Gbit/s throughput.

"People might have servers with older memory controllers," says Santanu Biswas, product manager of the Storage Components Group at LSI. Along with QLogic and Emulex, LSI makes 4-Gbit/s FC HBAs. He acknowledges that as cusotmers adopt 4-Gbit/s FC, they're tend to upgrade everything. "People are upgrading whole infrastructures. It's a cumulative process," he says.

But bus and controller issues aren't the only things that affect throughput. A move to 4-Gbit/s could put the focus on applications that have intensive I/O, such as enterprise databases. According to analyst Greg Schulz of the StorageIO consultancy, IT pros must switch gears to analyze I/O per second and response time in order to see whether a server even needs to push data at the level of 4-Gbit/s networks.

"To run at full 4-Gbit/s... you need a server that has a CPU and I/O chipset that supports at least PCI 2.1... or better yet non-bridged PCIe from CPU chipsets to the backplane; adapters capable of full 4-gig; switches capable of 4-gig; appropriate cabling; a storage system with 4-Gbit/s host ports; and enough internal power to drive 4-Gbit/s bandwidth without bottlenecks to or from the disk drives -- which may also mean 4-Gbit/s disk drives," Schulz maintains. "Of course, all of that is for naught if the applications on the server cannot generate or don't need 4-Gbit/s bandwidth!""The real question is whether or not 4-gig is necessary for most servers," says another analyst, who asked not to be named. "The vast majority of servers will be adequately serviced by 1-Gbit/s iSCSI or 2-Gbit/s FC."

If servers do need to be upgraded, it may not be a huge cost. The latest IBM xSeries servers start in the $2,000 range, and according to IBM spokeswoman Holly Parker-Coney, "100 percent of IBM's System x servers are equipped with 400 MHz or greater FSB."

There may be another boon to upgrading servers. According to Schulz, the kind of server internals required to support 4-Gbit/s FC also can support 10-Gbit/s Ethernet and InfiniBand interconnects.

As Andy Tran says, it's about being ahead of the game.

Mary Jander, Site Editor, Byte and Switch