Saturday, August 1, 2009

To do iSCSI or Fibre Channel? (or FCoE)

Well, there is always a discussion of the connectivity type when somebody says "SAN". Before iSCSI became popular, SAN means Fibre Channel connectivity; and now, when people start talking about SAN it could be anything they might be referring to. In order to avoid any confusion, please make sure which particular protocol is being referred to while starting the discussion.

The heated debate an year ago (and possibly still going on) was that FC will be phased out or as some went as far as saying that Fibre Channel will become obsolete within few years and that iSCSI will "rule". Those statements, I would say might be the case for very-small to small SAN implementations where there is no hard requirement for having high throughput and low latency block disk access. We all agree iSCSI can do what FC does, but the real difference comes in what exactly the application requirement is and what type of application it is.

Even before the heat on iSCSI is gone, now everyone touts FCoE as the technology that will make FC obsolete in the next few years. I am not sure exactly how that can happen, but we will have to wait and watch. Each technology has its own merits and de-merits. One has to apply them to their particular environment before making any decisions - rather than using someone else's judgement.

Pro's and con's of each technology
  • Fibre Channel is the best and (my) preferred connectivity for block based access in a SAN. The more obvious reason being connection reliability, delivery assurance, end-to-end manageability and control. It is not absolutely required to have high-speed networks for replication with FC (obviously asynchronous) as it can be achieved over high-latency, low-bandwidth networks, and even over the internet using vpn tunnels using FCIP. The only drawback to this is the investment for FCIP modules / FCIP routers (if you already do not own one). With most FC switch vendors you can either plug in a module that has both FC and IP ports in which case you are not sacrificing FC-port count or get a modular switch with both FC & IP ports. In case of any troubleshooting, all you need is the SAN/Storage Engineer and/or the System Admin which makes it quite easy for fixing problems.
  • The reliability and delivery guarantee is 'ok' with iSCSI, and it depends on the network in place, connectivity speed, whether using software initiator or hardware ToE. Dedicated network switches are recommended for using iSCSI for better performance and reliability and to not have more than 1-hop. The advantage in using iSCSI starts with cost/procurement and in-house knowledge-base. All companies have an IP network and a Network Engineer to start with. At the max, it will be required to purchase couple of more switches and some cabling and viola - you have a SAN. This is where the juice is for most companies as it is very expensive to invest in a Fibre Channel setup. But as the network grows, and iSCSI usage increases, speeds become a concern - even with 1Gbps network; at which point an investment has to be made in 10G network. Overhead on the servers (if using software iSCSI initiators) need to be considered and packet loss / TCP retransmissions will be an issue if going over more than 1-hop. Not soon, you will start seeing application performance degradation.
  • I am not sure what would be the reason to go with FCoE (maybe I am missing something), but at least it appears that this is the latest buzzword eveyone seem to be interested in. With FCoE investment will be same as in a Fibre Channel setup - and maybe even more. Since this technology is an upcoming one, the CNAs (Converged Network Adapter) and the switches that support it will be expensive. Also, all of this need to be 10G to have the bandwidth of an equivalent 4G/8G FC SAN. This is defnitely an improvement over iSCSI where the overhead of converting FC frames into TCP is eliminated and with Ethernet speeds at 10G and maybe 20G/100G soon in the future, there should not be any worry about the bandwith. What particularly comes into mind when comparing FC and Ethernet technologies is the latency. I am pretty sure FC latency (light speed) cannot be matched with copper transmission latency.

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