Internet Small Computer System Interface (iSCSI)

How iSCSI Works

When an end user or application sends a request, the operating system generates the appropriate SCSI commands and data request, which then go through encapsulation and, if necessary, encryption procedures. A packet header is added before the resulting IP packets are transmitted over an Ethernet connection. When a packet is received, it is decrypted (if it was encrypted before transmission), and disassembled, separating the SCSI commands and request. The SCSI commands are sent on to the SCSI controller, and from there to the SCSI storage device. Because iSCSI is bi-directional, the protocol can also be used to return data in response to the original request.

The iSCSI storage system is usually referred to as a target and is accessed by other systems across the network via iSCSI initiators; these can be either software initiators embedded within the host operating system or hardware initiators in the form of host bus adapters (HBA’s). The benefit of using hardware initiators is their ability of some HBA’s to offload the TCP/IP activity on to the card thus reducing the overhead on the host CPU. These HBA’s can also be referred to as TCP Offload Engines or “TOE cards”. 

Benefits of iSCSI

iSCSI deployments are typically within tier 2 and tier 3 data centres in large organisations, in the core of data centres for small/medium enterprises and also in remote offices. iSCSI has grown quickly  and is usually chosen for environments that Fibre Channel has had difficulty penetrating due to cost, complexity, functionality or support issues.

The application sweet spot has been storage consolidation and virtualisation for business critical Windows environments. Its popularity has not only been driven by the fact you can deploy robust affordable SAN solutions on x86 server platforms, but also because even entry level iSCSI disk arrays usually include sophisticated data management capabilities such as point in time copy, remote copy, thin provisioning and asynchronous mirroring. It is these features that allow organisations to significantly improve storage provisioning, back up and disaster recovery whilst reducing infrastructure and management costs. Besides product features, other benefits include ease of management, unlike Fibre Channel, which needs dedicated support, iSCSI being IP based means current server and applications administrators usually have no problems configuring and maintaining and iSCSI based SAN. 

iSCSI has a bright future ahead of it and it will continue to grow and take share from the entry level Fibre Channel market. Current providing acceptable performance on 1GbE, iSCSI will really show its true performance benefits once 10GbE is adopted as standard within data centre and server environments that utilise multi core CPU’s to deliver bandwidth hungry applications such as virtualisation and video streaming.