How-To: Plan an iSCSI SAN
February 10, 2006
Make sure to use the latest manufacturer-specific drivers for whichever NIC you choose. The generic drivers that come with Windows usually don't support advanced features like jumbo frames and TCP checksum/off-load.
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A TOE card from Alacritech, Chelsio and others goes a step further. Its on-card processors perform TCP segmentation and reassembly, as well as checksum calculations. Plus, TOEs can accelerate any type of TCP traffic and work with the same software initiators as other Ethernet cards. An iSCSI HBA, such as those from QLogic and Adaptec, off-loads not only the TCP management but also the higher-level iSCSI protocol. It looks like a disk controller to the host operating system rather than an Ethernet card.
Although TOEs and iSCSI HBAs can save your server a few CPU cycles--up to 10 percent or 15 percent running common applications like SQL Server--our experience is that they don't live up to their vendors' promises of faster disk I/O. And most midrange servers aren't CPU-bound, so we only recommend TOEs and HBAs for those rare servers that are.
The big advantage of an iSCSI HBA is it makes booting from the iSCSI SAN easy. Because HBAs act like disk controllers (complete with INT13 BIOS support), you can put your system drive on an iSCSI target. Booting from the SAN makes creating multiple similar servers easy: Just copy the boot volume to create a tenth Web or terminal server, for example. And it's easy to replace a failed server by attaching its volumes to a spare blade or 1U server of the same model.