Lock-in or escape
There are two ways to extend access to Fibre Channel storage area networks (FC SANs) to servers without FC host bus adapters (HBAs): Fibre Channel over Ethernet (FCoE); or iSCSI. Both use Ethernet as the transport medium but one locks you in to Fibre Channel and the other provides an escape route.
With iSCSI, SCSI storage commands and data are wrapped up inside TCP/IP data packets and sent across Ethernet to a storage resource. This could be a pure IP SAN, such as those from Dell/EqualLogic and LeftHand Networks or it could be a Fibre Channel SAN via an iSCSI port on an intervening FC fabric switch or director.
It seems to be the case that IP SANS are a more popular destination for server iSCSI ports than FC SANs. Such IP SANS are less expensive to set up because you don’t need a Fibre Channel fabric to access them and the accessing servers can use Ethernet NICs (network interface cards) instead of more expensive HBAs.
However, as soon as the latency and reliability of data access becomes more important an IP SAN may have disadvantages. The TCP/IP processing takes up server CPU cycles. That can be dealt with by using TCP/IP Offload engines (TOEs) which add TCP/IP processing hardware to a NIC and offload the host CPU. Naturally the NIC becomes more expensive.
A second problem is that mixing specialised storage traffic with generalised Ethernet traffic can slow down both sorts of traffic. One advantage of a Fibre Channel SAN is that it has a dedicated server-storage highway,
There is a more fundamental problem in that Ethernet isn’t a guaranteed lossless medium and nor does it have predictable latency for packet transmission. Bear this in mind as we look at FCoE.
With FCoE FibreChannel storage access commands and data are packaged for transmission across Ethernet rather than Fibre Channel. This requires another extended NIC and it is called a CNA, a converged network adapter. Cisco, Nouva, Brocade, QLogic and Emulex all made FCoE announcements and demonsrated FCoE kit at last week’s Storage Networking World in Orlando, Florida.
These companies all referred to the comming lossless and predictable data delivery version of Ethernet generally known as data centre-class Ethernet. The idea is that FCoE and data centre-class Ethernet would provide the equivalent of a FC connection for servers without the exopense of the having HBAs or extending a Fibre channel fabric to cope with many more servers accessing it.
FCoE gives you the option of using Ethernet as an access route into a Fibre Channel SAN but nothing more. The iSCSI route gives you the option of accessing a Fibre Channel SAN or an all-IP SAN. Look; no Fibre Channel.
Data centre-class Ethernet will benefit iSCSI as well as FCoE. It will give IP SANs a lower latency and more reliable access path to and from accessing servers just as it will do that for FCoE.
On the grounds of flexibility and choice iSCSI looks to have a significant advantage over FCoE. We can expect IP SAN vendors to be eagerly awaiting data centre-class Ethernet because it gives them the wherewithal to promote IP SANs into the enterprise big-time.
It may be that FCoE has advantages of its own that iSCSI cannot match. We’ll wait for them to be revealed as the marketing struggle between iSCSI and FCoE gets under way.