Do you really need an iSCSI host bus adapter (iSCSI HBA)? The short answer is no, with a but. It is possible to build a working iSCSI SAN with all of it features using standard Ethernet network interface cards (NICs) as the initiators, but it might not be the best plan to have an iSCSI SAN with out an iSCSI HBA
Older versions and models of iSCSI SAN's needed iSCSI HBA's because the SAN's would perform poorly without iSCSI HBA's. The two issues with iSCSI SAN's that do not use HBA's are performance and booting.
One big variation between an iSCSI HBA and a conventional NIC is that the HBA includes a TCP Offload Engine (TOE), which is an support processor that takes care of wrapping and unwrapping SCSI information in Ethernet frames. Since handling iSCSI maintenance can easily eat 20 percent or more of the CPU cycles of the iSCSI server, the server in an iSCSI SAN that does not use an iSCSI HBA
may show a loss in performance.
Nevertheless, the trend toward increasingly powerful servers at reduced costs means that more servers are likely to have the CPU cycles to spare. This is especially true in environments that do not virtualize their servers.
In relation to boot issue, an HBA initiator lets the system boot from the SAN. Since the system can't handle iSCSI packets until it is up and running, an iSCSI SAN without an HBA must boot from a directly attached hard drive or other device.
However, there are now ways to boot from an iSCSI SAN without HBAs. For example, NetBoot software from iSCSI HBA
enables booting by storing a copy of the system's boot volume in the iSCSI array and using a Windows boot initiator on the system driver to load the boot files from the SAN at startup.
There is one last factor to consider. Originally, most iSCSI SAN's were alternatives to, or replacements for, Fibre Channel SAN's. They were not expected to offer the same performance as FC SAN's -- early iSCSI SAN's didn't have the same throughput as FC SAN's and even today a high-performance SAN is more than likely an FC SAN.
However, the low cost of iSCSI SAN's is increasingly encouraging enterprises to use them in areas that do not require the performance of an enterprise SAN, such as snapshotting or for departmental computing.
Conclusion: In some instances it is worth considering an iSCSI SAN without an iSCSI HBA. But it will take analysis, and probably some testing with performance tuning tools such as iostat, to see if you can get by without iSCSI HBA's in a particular application.
To learn more about iSCSI SAN's and iSCSI HBA please visit SANRAD at http://www.sanrad.com/iscsi.asp?IP-SAN=229
