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By | Chris Mellor 20th August 2010 16:04

Ethernet storage protocol choices

AoE, iSCSI or FC?

You The Expert: We're moving to a world in which all storage network protocols run over Ethernet. That means both file and block. There's no dispute about running file access over Ethernet but the block world is squaring up for a three-way tussle.

The three block protocols are iSCSI, Fibre Channel and AoE. iSCSI uses the TCP/IP stack to provide lossless delivery of SCSI packets of commands and data between servers and storage arrays. Ethernet operates at layer 2 of the 7-layer OSI model and TCP/IP operates above that at layer 3.

An iSCSI link requires a server to have an Ethernet NIC (network interface card). Before storage request and data traffic passes out through the NIC it has to be packetised by the TCP/IP stack and this can be done by the host CPU, using cycles that could otherwise run application code, or by an enhanced NIC with a TCP/IP Offload Engine (TOE) implemented on it, saving server host CPU cycles but adding cost.

Fibre Channel

Typically iSCSI has been positioned as a non-business-critical storage area network (SAN) protocol. The critical applications access their networked storage across Fibre Channel (FC) fabrics, FC being the second block access protocol, and FCoE (Fibre Channel over Ethernet) runs this protocol across an enhanced Ethernet, one that is lossless.

Physical Fibre Channel requires a server to have a special Host Bus Adapter (HBA) which constructs Fibre Channel frames and sends them out across the wire, also receiving incoming frames and unpacking them.

With FCoE a standard Ethernet NIC has to be upgraded to a CNA, a Convereged Network Adapter, that provides both Ethernet interfacing and FCoE processing. FCoE requires special CNAs and DCE-class Ethernet using, it is assumed 10GbitE wire, whereas iSCSI requires ordinary NICs or NICS with TOE functionality, and runs across ordinary Ethernet on 10 or 1GbitE wire, or even slower wire if costs are really constrained.

The net of this is that iSCSI links should be cheaper to set up than FCoE links, however they involve layer 3 processing whereas FCoE does not.

Data Centre Ethernet (DCE) provides lossless transmission of FCoE frames (inside Ethernet jumbo frames) at layer 2 without needing any layer 3 functionality.

Does such a lossless Ethernet render iSCSI a waste of network overhead? All those layer 3 operations dedicated to not losing packets are rendered redundant if iSCSI is run across a DCE wire.

We are not looking at the routing issue here; that is separate from iSCSI or FCoE running across Ethernet.


The third Ethernet block access storage protocol is AoE (ATA over Ethernet), the protocol Coraid is championing. It is in start-up mode, not having the wide market penetration enjoyed by iSCSI and Fibre Channel. AoE requires neither TOE nor CNA, ordinary NICs sufficing.

ESG has given it a brilliant endorsement, stating: "AoE is a simpler and more direct protocol than either iSCSI or Fibre Channel. AoE is not built on IP, TCP, or SCSI; packets are addressed to devices using their Ethernet MAC addresses and sent across the network with a minimum of overhead."

"Both Fibre Channel and iSCSI run SCSI over high level networking protocols on top of a physical network infrastructure, consuming additional overhead and processing compared to AoE, which connects servers and storage directly across the physical Ethernet layer."

We are left with several perplexing questions.

Does AoE offer the simplest Ethernet storage of all, providing the best possible base for both block access, rendering both iSCSI and FCOE unnecessary? Does its start-up nature and relatively low market acceptance make it a risky choice and so rule it out?

Does FCoE running on data centre class Ethernet mean iSCSI is no longer needed? Can iSCSI provide the same performance and reliability as FCoE for less cost, meaning FC and FCoE can be replaced by iSCSI?

Should large data centres consider running both iSCSI and FCoE with any eventual convergence onto a single protocol put aside for later consideration?

Is there a case to be made for running AoE alongside iSCSI and/or FCoE?

Does a choice of AoE lead to pure AoE shops with no need for iSCSI or FCoE?

We are going to ask our reader experts for their opinions on where Ethernet block storage access protocols should be used and which ones preferred. If you reckon you have answers to any of these questions that are solid as a rock and can withstand detailed assault by proponents of alternative views then get in touch. ®

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