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Converging SAN traffic in the data center

By David Klebanov, technical solutions architect, CCIE #13791, Cisco Systems, Network World
December 13, 2011 11:32 AM ET

Network World - Klebanov is a technical solutions architect with Cisco Systems. He has 15 years of network industry experience. In recent years he has been closely involved with architecting data center solutions. He can be reached at klebanov@cisco.com or on LinkedIn at http://www.linkedin.com/in/davidklebanov

Most organizations today use dedicated storage networks in the data center, but the concept of leveraging converged network infrastructure to provide organizational storage services is gaining steam.

In most cases NAS storage traffic already rides existing network infrastructure in a converged manner, so in this article we are going to focus on converging SAN traffic.

SAN/LAN convergence can pay dividends today

Traditional SANs define the concept of a fabric where initiators (servers) are connected to targets (storage arrays) through a purpose-built fiber infrastructure, which consists of SAN switches using Fibre Channel protocol for encapsulating and forwarding the SCSI traffic hop-by-hop and end-to-end. One way to make use of converged network concept is to have SCSI traffic encapsulated in a protocol that can be transported over existing IP networks.

TCP/IP provides a very convenient over-the-top way of deploying SAN environment in such manner. SCSI protocol transported using TCP/IP is called iSCSI.

Using iSCSI has several main drawbacks:

• An iSCSI capable storage array is required. An alternative is to use a storage switch that can terminate iSCSI TCP/IP connections, unwrap the SCSI portion and forward it in a native format toward a Fibre Channel connected storage array. In this case storage array is not required to support iSCSI.

• Over-the-top behavior of iSCSI prevents storage administrators from enforcing per-hop storage characteristics and controls, which is an inherent part of the vast majority of SAN deployments.

• The use of the TCP protocol makes iSCSI traffic susceptible to TCP slow-start mechanism because TCP cannot differentiate between data and storage traffic. iSCSI can leverage switching infrastructure that supports Data Center Bridging (more on that later), which can apply selective back-pressure, pausing iSCSI traffic and preventing it from being indiscriminately dropped during times of congestion.

Things like firewalls, IDS/IPS and application optimization can also influence the delivery of iSCSI traffic end-to-end, but on the up side, iSCSI allows us to extend the reach of a storage network as far as our IP infrastructure goes, which may be way beyond the limits of a dedicated traditional Fibre Channel SAN environment.

Fibre Channel over Ethernet

Another method of delivering SCSI connectivity between initiators and targets in a converged network fashion is to encapsulate it in Fibre Channel protocol and use Ethernet frames for transport. This technology is called Fibre Channel over Ethernet (FCoE). FCoE I/O and control traffic is differentiated from the regular data traffic by the means of ethertype values present in the Ethernet frame header fields.

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