The iSCSI protocol has a lot of built-in overhead because it encapsulates SCSI commands in TCP packets. By some tests, as much as 30 percent of a system's processing power can be eaten up by iSCSI overhead. For that reason iSCSI networking companies are offering HBAs with built-in TCP Offload Engines (TOE) to take the load off the system. In the last few months a number of companies, including Adaptec, Intel and Alacritech have brought such products to market.
While TOE-enabled iSCSI HBA's have major advantages, this is still a very new market with a lot of differences between products and storage administrators should make careful comparisons when choosing cards for their iSCSI SANs. Particularly important areas are CPU utilization and throughput. Although all these products take a load off the system processor, they do so to varying degrees and it is worth running some tests to compare contending products.
Storage administrators should also be aware that the first generation of products are still rough around the edges, particularly in ease of use. This is improving rapidly as manufacturers bring out new GUIs.
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.