Can iSCSI catch up with Fibre Channel?

The availability of Microsoft's free downloadable iSCSI drivers has led to suppliers providing tailored hardware as a...

The availability of Microsoft's free downloadable iSCSI drivers has led to suppliers providing tailored hardware as a cost-effective alternative to Fibre Channel for SMEs

A prohibitor to the large-scale adoption of iSCSI has been a lack of products supporting the iSCSI protocol. However, following the introduction of Microsoft's iSCSI drivers, iSCSI-specific products are now appearing.

These products are mostly from small, niche players, and suppliers such as HDS have been working on technology to transfer data faster using iSCSI. The larger suppliers claim to support iSCSI, but they are not generally producing iSCSI-specific products.

In iSCSI's favour is the fact it is a storage networking protocol based on IP, which was developed by the Internet Engineering Task Force and ratified as a standard in February 2003. Compared with some protocols, ratification was relatively quick, largely because iSCSI is based on well-established SCSI and TCP standards.

Using iSCSI, SCSI block storage commands are encapsulated by iSCSI into Ethernet packets which are transported over IP networks, enabling servers to communicate with shared storage devices over a standard IP structure using normal SCSI storage commands.

Servers are generally attached to local storage using a dedicated SCSI connection and a block-level interface. iSCSI uses existing Ethernet switches, cables and routers. Many of the limitations of SCSI are overcome by iSCSI as it supports faster speeds and longer distances.

It is because of these advantages over SCSI that iSCSI has been heralded as an alternative to Fibre Channel, but its performance does not yet match it. Fibre Channel has already reached a speed of 2gbps, with 4gbps due out around the middle of 2004. Suppliers are also looking at 10gbps. At the same time, iSCSI has only reached 1gbps over Gigabit Ethernet. A 10 Gigabit Ethernet is still a while away, but when it arrives, 10gbps iSCSI is being promised by suppliers.

Originally, Fibre Channel suppliers intended to move straight from 2gbps to 10gbps, but 4gbps has been introduced as an interim measure because of the high cost of 10gbps.

This provides iSCSI with a chance to catch up with Fibre Channel, dependent on 10 Gigabit Ethernet arriving before the Fibre Channel suppliers move to speeds in excess of that.

This will not suddenly lead to iSCSI being adopted in enterprise datacentres. Many large companies have invested too much in Fibre Channel to rip it out. Nor will it mean that organisations with iSCSI Sans enjoy better performance than those with Fibre Channel.

Many of the organisations with Fibre Channel Sans are in highly competitive industries where speed of performance can provide a real advantage. As soon as 10gbps Fibre Channel becomes available these organisations will adopt it to ensure they remain on a level playing field with their competitors.

In this way, Fibre Channel will always remain one step ahead of iSCSI, and therefore will be the technology of choice for many users. The cost of 10 Gigabit Ethernet will not be as low as some companies believe, and with the price of Fibre Channel cabling falling, the difference in price will not be as great as many think.

It is not just the cost advantage that makes iSCSI attractive. It is less complex than Fibre Channel, requires fewer changes to infrastructure and has no specialist networks.

Because Ethernet is widely deployed, there is less disruption and cost involved in implementing iSCSI, and organisations can leverage their existing investments to a greater extent than with Fibre Channel. These factors will make iSCSI attractive to some small and medium-sized enterprises, which cannot afford to lay Fibre Channel, and also do not have the expertise or the resources to implement complex infrastructures.

Because iSCSI is IP-based, it does not have the distance limitations of SCSI, and information can be transported across Wans or the internet. However, there may be some latency and bandwidth issues on longer distances.

Distances of several hundred kilometres can be supported with Fibre Channel and because it uses dedicated links it is less prone to bandwidth problems. Again, this makes iSCSI attractive to organisations that need to transport data over long distances but where speed is not critical.

iSCSI and Fibre Channel allow organisations to share storage resources across servers and easily expand storage capacity, which is what makes iSCSI suitable for use in IP-based Sans. An early benefit of Fibre Channel is that it provides guaranteed delivery at the lower layers of the protocol stack, although TCP does provide guaranteed in-order delivery.

With all of these advantages it may appear surprising there has not been a huge uptake of iSCSI among smaller organisations. This is because of the limited provision of support from suppliers.

When iSCSI first came on to the scene it was going to revolutionise storage and replace Fibre Channel as the main form of connectivity in Sans. Unfortunately, there was little support from suppliers and some of the early iSCSI products were withdrawn because of a lack of customers, reluctant to adopt a technology for which there were few products.

However, the introduction of Microsoft's iSCSI drivers in June 2003, which can be downloaded for free from the Microsoft website, have provided impetus for iSCSI and manufacturers are at last beginning to provide hardware with built-in iSCSI cards.

Although the speed of iSCSI is not yet sufficient to support business-critical applications, it will have a role to play in supporting data in the back-up of non-critical business applications that are connected to network-attached storage devices.

Where iSCSI will pick up custom is the lower end of the SME market, which will not be able to afford to implement a Fibre Channel-based San. Here, deploying an iSCSI-based San will provide a competitive advantage over companies that retain direct-attached storage.

Sue Clarke is senior research analyst at Butler Group

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