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Should ATA disks become a standard for online data?

ATA is great for archiving, but may not be able to replace Fibre Channel storage arrays.

Dr. Geoff Barrall
CTO, BlueArc Corporation
Dr. Barrall is the CTO, executive vice president and co-founder of BlueArc Corporation and the principal architect of its core technology, the SiliconServer Architecture. Prior to joining BlueArc, Dr. Barrall founded four other ventures, including one of the first Fast Ethernet companies and a successful UK consultancy business. In this role, he was involved in the introduction of innovative networking products into UK markets including the Packeteer and NetScout. Dr. Barrall received his PhD in Cybernetics from the University of Reading in 1993.

Many companies and vendors are excited about the prospect of enterprise ATA-based storage arrays and the effect these will have on decreasing costs in enterprise storage. Certainly by constraining storage access speeds, it's possible to build cost-effective ATA storage archives, as has been done by EMC in their Centera, and Network Appliance in their NearStore products. However, these products, while great for the archiving functions they have been designed for, are unlikely to replace entrenched Fibre Channel storage arrays due to their relative lack of performance.

In Figure 1, you can see a comparison of a typical array price of Fibre Channel storage compared with the costs of purchasing an equivalent ATA-based chassis for an online storage application. Things here are much as you would expect, with the ATA system costing less than the Fibre Channel system. However, things became more interesting as one compares the relative cost per MB of ATA storage versus Fibre Channel storage. As can be seen in Figure 2, the actual relative cost per MB of Fibre Channel storage compared with ATA-based storage decreases as the amount of storage increases.

At first glance, this seems counterintuitive as ATA disks have been touted as being very inexpensive for larger archives, so why this decreasing margin?

Two factors are currently hampering the cost of ATA storage. The first factor is that the ATA disks tend to sell for much lower margins than the Fibre Channel ones do. This means vendors have much more flexibility for bulk discounting with the Fibre Channel disks allowing them more flexibility to decrease in cost as the size of the purchase increases. The second factor is that ATA disks are still using parallel data busses for connectivity. This means it is almost impossible to daisy chain ATA disk chassis together in the way Fibre Channel chassis can, using a single cable for their serial interconnect. Thus, in a SAN environment, controlling logic must be purchased with each ATA chassis whereas Fibre Channel chassis may share this logic. This issue should be resolved with the introduction of SATA drives and chassis next year using new silicon from vendors like Sierra Logic and others.

So for now, while ATA remains strong in the backup and archiving markets, for online data the cost differential between Fibre Channel and ATA storage per MB remains close (although it's worth remembering that the actual dollar amount saved could be many $10,000's on a large purchases). Further development in the ATA space and the adoption of SATA will change these dynamics in the next 12 to 24 months for the first time allowing a breakthrough in price/performance for the enterprise data center.

Copyright 2002, Blue Arc Corporation.

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