A key issue around the development Converged Enhanced Ethernet(CEE) (also known as Data Centre Ethernet in Cisco marketing because of their proprietary extensions), is that CEE will enable iSCSI to reach it’s full potential. For nearly everyone, iSCSI will become the default the technology once the CEE standards are finalised, and products come to market. What ? You haven’t heard this before ? Lets have a quick intro to CEE.

What are we trying to solve ?

Existing applications are tolerant to packet loss in the network, and QoS is managed (but not solved) by classifying traffic and managing queues in the network (where queues exit). However, Storage is very sensitive to loss, both FC and iSCSI can retransmit lost data, but this causes significant bottlenecks and a performance hit to the application.

In a modern data centre, attempting to QoS storage AND voice AND other high value traffic is a practical impossibility. The configuration and maintenance of a converged backbone with competing requirements would be unsuccessful in enough cases that the market would reject any such attempt.

The obvious thing is to stop packet loss. The only practical way to achieve this is to create some signalling that notifies the sender to stop sending data before the loss or congestion occurs. Congestion is always possible in ANY NETWORK no matter how much bandwidth you provision.

IEEE are working on 802.1Qau Congestion Notification which is an end to end message. When combined with 802.3x Pause control (which operates at link level), we can guarantee zero loss in Ethernet backbone because the source can be signaled to slow down the data transmission.

Note that 802.3x needs to be modified from a start/stop for the entire link, to be able to Pause certain traffic priorities, so when combined with 802.1Qaz you are able to overcome this limitation. 802.1Qaz describes Enhanced Transmission Selection which support allocation of bandwidth amongst traffic classes.

Another standard  under development is Discovery and Capability Exchange where servers and network equipment are able to signal their capabilities to each other, so that strategies for traffic management can be agreed between all elements.

So now we have the ability to configure our network with at least three major types of buffering or congestion strategies. 1) zero loss suitable for voice traffic 2) buffer overflow causes signal to be sent back to source or upstream network device such traffic is controlled and not dropped 3) discard traffic if needed and let application protocols retransmit. (of course, you should be able to have less, none, or variations in the same way that we do today.

Unlike using TCP/IP to perform QoS, where the QoS mechanisms are not will suited to storage traffic, CEE will deliver a zero to low loss switching backplane to iSCSI and yet offer support to a number of other applications. The option for single, unified Ethernet fabric in the Data Centre is very real.

Further, we will still have native IP support for WAN for inter-Data Centre connections when needed without extra actions or work. (Unlike Fibrechannel, which will still need FCIP plus FCOE / FC to perform inter-Data Centre connections).

Conclusion

The interesting outcome of this is the impact on iSCSI. Storage network technologies are very sensitive to packet loss, and if the network can assert mechanisms for assuring low latency and very low (or no) loss, then our Converged Enhanced Ethernet Networks will take over the data centre for our storage.

Most importantly, iSCSI will now be able to reach it’s full potential. Most people find that iSCSI works fine in Small to Medium server farms, but in large scale IT the impact of packet loss is too great a risk. By comparison Fibrechannel networks deliver a lossless medium and that is a key enabler for the technology.

Look for my next post which discusses more aspects of Converged Enhanced Ethernet and its impact on Storage networks.

References

“Storage convergence over Ethernet: iSCSI for new SAN installations FC tunneled over Ethernet (FCoE) for expanding FC SAN installation” Data Center Bridging, IEEE 802 Tutorial – Page 11 – 12th November 2007.

http://www.ieee802.org/1/files/public/docs2008/az-wadekar-dcbcxp-overview-rev0.2.pdf