Ethernet Extender

A better way to adopt converged network concept for SAN is to have SCSI traffic encapsulated in a protocol that can be transported over existing IP networks, says David Klebanov,  technical solutions architect, Network World (News - Alert).

In traditional SANs, 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.

By leveraging TCP/IP, a convenient over-the-top SAN deployment can be performed. The SCSI protocol transported using TCP/IP is known as iSCSI.

iSCSI, however, has several drawbacks. It requires an iSCSI capable storage array. Alternatively a storage switch can be used to 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.

Further, 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.

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

In addition to these, Klebanov says, firewalls, IDS/IPS and application optimization can also influence the delivery of iSCSI traffic end-to-end.

Despite these drawbacks, iSCSI allows users to extend the reach of a storage network as far as their IP infrastructure goes. This according to Klebanov, may be way beyond the limits of a dedicated traditional Fibre Channel SAN environment.

A recent TMCnet report finds that the market for Ethernet equipment amounted to over $16 billion in 2010, while the resource of IPv4’s 4.3 billion addresses is almost exhausted. This provides great opportunity for vendors that deliver Ethernet extenders.