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Go slow on TCP/IP offload engines

Why you should take a cautious approach.

The TCP Offload Engine (TOE) promises an enormous speed increase for storage networks that use the TCP/IP protocol and Ethernet. However, they're still very new and storage administrators may want to proceed cautiously, especially if they are making the decision on which storage network protocol to use based on the performance promises of TOEs.

One of the drawbacks of Storage over IP for SANs is the overhead involved in handling the TCP/IP protocol. Unlike the Fibre Channel protocol, TCP/IP was designed to send information over long distances over relatively unreliable links and it includes a lot of overhead to make sure the information arrives uncorrupted. The downside is that it takes significantly more computing power to get equivalent throughput to Fibre Channel even if the communication channel is equally fast.

A TOE takes the job of translating all or part of the TCP/IP protocol away from the main processor and gives it to a specialized processor, often on the network interface card (NIC). The result is higher throughput. Two of the early makers of TOE card, Alacritech and Adaptec say they have demonstrated iSCSI data transfer rates in excess of the current Fibre Channel rates of 200M Bits per second. Typically a TOE also reduces the load on the system processor to less than 10 percent, compared to as much as 100 percent of the processing power with some systems without a TOE. (Intel, which recently entered the TOE market, says that a Pentium III server running iSCSI without TOE can expend nearly all its processing power handling the protocol translations. With its PRO/1000 T IP Storage Adapter, Intel claims the main processor load is less than five percent.)

While TOEs look like a winner, and you can buy them today for prices around $700 to $1,000, the market for them is still shaking down. That means the usual uncertainties for early adopters and the cautions that go with them. For instance, while the products work, as demonstrated by several public tests, there are still very different approaches among the manufacturers and it's not clear which one will ultimately win out. One measure of the novelty of all this is that the Xiran division of Simple Tech doesn't even call its product a TOE, preferring the term "content delivery adapter" to identify it with the other hardware-based accelerators it manufactures.

One question still being thrashed out is how much of the TCP/IP conversion to do in hardware and how much in firmware. Some companies, such as Adaptec and Xiran, do everything in hardware because it minimizes the load on the system processor. Others, such as Alacritech, do a partial off-load, handling things like the connection in software and only using the hardware for the data conversion.

The 10 Gigabit Ethernet Alliance has a white paper on TOEs titled "Introduction to TCP/IP Offload Engine (TOE)" at its Web site.

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.


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