Issue TOC Print full article Print this page Download as PDF E-Mail this URL Discuss this article Flame the author     In this article Introduction Products Reviewed Executive Summary How We Tested Online Only: Multiple Target Woes Report Card

During our iSCSI tests we learned that there's often a problem reading a single target. To make sure we have the terminology straight, we define a target is a device you can partition into multiple LUNs (logical unit numbers; in other words, what happens when you partition your local hard disk into multiple drives). For example, we split our Eurologic SANBloc 2 into two targets, each running at RAID 0. We then partitioned each target into two LUNs. In Windows, this looked liked four drives--E, F, G, and H. E and G were primary partitions; F and H were extended partitions on each target, respectively.

In contrast to every other storage protocol, read operations for iSCSI are more difficult to perform than writes. Incoming streams need to be reordered and processed, while that operation is handled on the target end for writes. Cards that perform TCP off-load tend to become bound by their internal CPUs, where the base NIC in the Dell 2650 had a pair of 2.6-GHz Xeon processors to work with.

The Alacritech and the Adaptec cards both performed well. The Alacritech uses a parallelized processor to handle TCP streams, meaning that it is much more efficient when handling multiple streams than a single stream. With only one target, you get only one TCP stream, and the onboard processor becomes a bottleneck--specifically, to the tune of 85 MB per second with iSCSI on the Alacritech card. On the Adaptec card, read performance was only 64 MB per second with a single target but rose to 87 MB per second with two targets. The Intel PRO/1000 T IP Storage Adapter card performed the worst, getting only 55 MB per second with two targets and about the same with one.

On-board CPU performance is the root cause of the slower performance for both the Alacritech and the Adaptec cards. At the time of this writing, we're unsure of the cause of the performance problems with the PRO/1000 T; Intel is investigating the issue.

So, are multiple targets the norm? In iSCSI's early markets--small departmental and small-business SANS and distance applications--a single target is likely, but as iSCSI is adopted into larger installations, the likelihood that multiple targets will become the norm increases greatly.