The Need for Speed
iSCSI does have some drawbacks. Current iSCSI technology is limited to 1 Gbps--the speed of Gigabit Ethernet--while Fibre Channel solutions have a maximum transfer rate of 2 Gbps (4-Gigabit FC is on the immediate horizon). We expect that iSCSI eventually will be deployed on 10 Gigabit Ethernet, but for now, that's not possible--there isn't a TCP/IP off-load engine capable of handling those speeds.
In three to five years, when 10 Gigabit Ethernet becomes more prevalent, you'll see iSCSI follow suit. But if your current requirements for responsiveness and backup speed can't be met at 1 Gbps, iSCSI is not the technology for you right now.
Similarly, if you've already invested a great deal in Fibre Channel technology, you may want to think seriously about making a considerable iSCSI investment. There is a definite case for using iSCSI in a limited capacity for distance applications and in less speed-critical situations. It can also be an inexpensive way to move forward if you are currently at capacity on your Fibre Channel switches.