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