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iSCSI SANs: Page 2 of 6

iSCSI Concerns

Remember this: You shouldn't expect to run your SAN network on the same generalized TCP/IP network that carries data between servers and users. Storage is simply too important to be on the generalized data network--it must be segregated onto separate VLANs or an entirely separate switched network. (There are exceptions to that rule, as in the case of a small workgroup using iSCSI for limited access to a main storage device or accessing storage remotely.)

Another consideration: iSCSI places a large burden on a server's CPU. iSCSI is essentially a wrapper around standard SCSI commands. Those commands must be received in order, and each command or data packet must be wrapped (or unwrapped). This creates a huge amount of TCP/IP work that the standard NIC card passes off to the CPU.

The solution is a TCP Off-load Engine (TOE) or an iSCSI Host Bus Adapter (HBA). These specialized cards perform all the TCP/IP overhead processing with on-board processors instead of passing it to the CPU the way standard NIC cards do.

TOE cards and iSCSI HBAs each have advantages and drawbacks. Whereas iSCSI HBAs deal only with iSCSI traffic, TOE cards can handle all standard Ethernet traffic. However, iSCSI HBAs use fewer main CPUs than do TOE cards for iSCSI operations, and they generally cost a few hundred dollars less than TOEs.