Analysis: Most Vital Part Of Virtualization May Be A SAN

THINK FAST
So what do you look for in a SAN to complement your virtual servers? Think wide striping across multiple drives to spread your data across numerous physical disks, resulting in faster response. The actual disk I/O is always the slowest part of system performance, and just like the disaster-recovery realization that occurs when you virtualize, the problem of disk I/O is more pronounced as you virtualize more servers. Try spinning up 20 or 30 database servers on one physical host pointing to one disk array--you're going to hit a bottleneck at the disk, negating the benefits you got by virtualizing.

Its not just spindle count you need to consider. For intensive disk applications, you'll need to make sure your SAN includes multiple controllers for better throughput management, as well as more redundancy to avoid a single point of failure. Other factors include virtualization/SAN feature support, server memory, drive speed, and technology platform--iSCSI vs. Fibre Channel.

Not all SANs are equal when it comes to working in the virtualized world. Microsoft, VMware, XenSource, and even Apple are working with SAN manufacturers to ensure that virtualization features are supported. Vendor certifications and formal partnerships are the norm, so make sure your storage and virtualization vendors are talking.

Disk performance also can become a factor when a server doesn't have enough memory to properly run its VM load, as the added reads and writes caused by swapping result in bottlenecks.

Our advice: Pony up for plenty of memory. If you don't have enough, you'll be relying on the disk subsystem's swap file to bring cached info from the disk back into memory. More trips to the disk equals slower performance. Also look hard at the speed of the drives themselves; this is definitely part of the performance equation.

The last issue, choosing iSCSI or Fibre Channel, is one of those debates that's becoming tiresome. Before joining the fray, make sure you address the design elements discussed above. They're more important. Then look at throughput versus interconnect speeds. A 1-Gbps Fibre Channel interconnect can deliver potential throughput of 96 MB per second, while a 1-Gbps iSCSI interconnect delivers 116 MBps. A 4-Gbps Fibre Channel SAN still delivers the highest throughput, but with 10-Gb iSCSI and 8-Gb Fibre Channel on the way, don't expect this debate to be over soon ... and don't get sucked in by it, either.

What often gets skipped over in the yelling is the most critical factor in improving performance: The design of your storage array. The more drives you have, the more spindles there are to read and write data from, the faster your data access. Add the optimum speed drives, the correct number of controllers, and ensure that you've got ample memory in your servers and you'll be fine.

And you're not limited to one or the other. Many orgs are using varied technologies and SAN designs based on the performance needs of the virtualized applications.

"When we started three years ago, our game plan was always to tie it to some type of SAN," says Manny Silva, director of IT for Kuehne & Nagel North America, of the logistics company's virtualization initiative. "We already had Fibre Channel SANs in place but actually opted to go with iSCSI for the virtualization. In our testing, the performance of both matched up."

The ability to mix and match your SAN environment lets you spend big on critical performance areas but go with a lower-cost option for disk backup or failover. Vendors like EMC, EqualLogic, and Hitachi have even designed "virtual storage" that lets you mix platforms on the back end while presenting a uniform front end for servers and applications. For sites that haven't yet deployed SANs, consider investing some of the virtualization savings realized in the server room. You'll be glad you did.

Michael Healey is chief technology officer of GreenPages Technology Solutions.
Write to him at [email protected].

Photograph by Jupiterimages

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