Over the past year iSCSI has picked up significant steam, particularly in regional and remote Datacenters which contain a large number of DAS. To date the majority of implementation involve primarily Windows servers even though Linux has lately shows as the next iSCSI frontier.
For those of you who may not be aware, there are several ways to implement iSCSI. First and foremost you need an array with Native iSCSI capabilities or an FC-iSCSI gateway in-front of the array that will convert FC frames into iSCSI packets. Needless to say there's some overhead associated with the latter approach since there's protocol conversion that has to occur in the gateway.
On the host side, there are several choices:
1) Most Operating systems vendors offer iSCSI software initiators free of charge than be used in conjuction with a regular NIC card to implement iSCSI. Windows, Linux, Netware, HP-UX, AIX and Solaris 10 offer such software initiators. These initiators are extremely easy to deploy and have no real cost since most servers today ship with at least 2 Gigabit Ethernet ports. One of the potential drawbacks of this implementation is the CPU overhead due to the TCP/IP and iSCSI processing. I say "potential" because in my 3 year experience with iSCSI i've seen exactly one occurrence of this and it was directly related to the age of the server, the number of CPUs, and more importantly the CPU processing power. To address situations like this, the 2nd iSCSI implementation choice is via the use of iSCSI TOE cards
2) iSCSI TOE (TCP/IP Offload Engine)cards are specialized cards that are used with the iSCSI Software initiator on a host and provide TCP/IP offload from the host's CPU to a chip on the card. This allows the CPU to focus mostly in processing application requests. TOE cards can also be used as regular NICs in addition to servicing iSCSI traffic. However, with the TOE approach all iSCSI processing still occurs on the host CPU. That's where iSCSI HBAs come into play. The average price for a dual ported TOE is around $600-700.
3) iSCSI HBAs are similar to FC HBAs in that they offload all protocol processing overhead from the host CPU to the card itself, thus allowing the CPU to focus entirely on application processing. The average cost of a single ported iSCSI HBA is somewhere between $500-600.
Which one of the methods you choose to implement is strictly dependent on your servers and more importantly the number of CPUs as well as the CPU processing power. For new servers, the last 2 approaches maybe a stretch since modern servers have a tremendous amount processing power and thus any overhead will most likely go un-noticed. However, for older servers, or for servers whith a current CPU utlization of > 70% then deploying a TOE or an iSCSI HBA will make sense.
The nice thing about iSCSI is that it offers choices, and flexibility in that it allows folks to reap the benefits of networked storage in a cost effective manner.