DAS VS. SAN

Remember the Storage Area Network vs. Network Attached Storage debate? There were books written about it, articles and forum debates (this is before we had blogs). Eventually NAS vendors like NetApp become SAN vendors and SAN vendors either created their own gateway NAS front ends as did EMC or partnered with someone that offered a NAS Gateway like ONStor or BlueArc. Now there is a new debate; Direct Attached Storage vs. Shared Storage.The DAS vs. SAN debate will end up much like the SAN vs. NAS debate; a middle ground will be found where the users will choose what makes the most sense for them and there will be integration between the two worlds that will allow for a seamless transition between them. Our next series of entries will examine the DAS vs. SAN debate and how to select the right strategy for your environment, or how to know when mixing them makes more sense.

If you have more than a few servers why would you stay with DAS at all? Most customers who stick with a DAS strategy do so because it has the reputation of being both inexpensive and simple. Those are big motivators not to network your storage. That said, it seemed like a foregone conclusion that as a business grew it would eventually network its storage to be able to improve utilization, data protection and ability to fully leverage server virtualization.

Despite these motivators to network your storage, inexpensive and simple were still big drivers to allow DAS to maintain a sizable market share, even more so DAS seems to be increasing its value proposition. There are three primary drivers for this. First very high storage capacity servers and some in the 20TB plus range now, second PCI-E based Solid State Storage and third medium capacity, high performance (450GB 15K RPM) SAS based drives.

As a result DAS has been able to keep pace with the demands of many growing businesses. The first driver for this is capacity. For many customers one of the reasons to implement a SAN was to gain access and to leverage the high capacity offered by shared storage systems. Today's servers can be configured with TB's of capacity suitable for many applications or virtualization environments.

The problem is that because the capacity of even the lowest capacity drive when put into a four or five drive RAID configuration results in far more overall capacity than is required by the one application running on the server. Also small capacity drives are not as cost effective on a price per GB basis as high capacity drives. This is what leads many customers to a SAN; to leverage high speed, high capacity drive economics across multiple servers as to not waste the resource but yet optimize the investment.

In the past a SAN was the only way to accomplish this balancing act, but now this is where the second driver comes in. PCI-E SSD cards from suppliers like Texas Memory Systems and Fusion-io allow for very high performance, application specific storage to be built and to be done so at a smaller capacity. The final case where large capacity performance and reliability are needed can be resolved with the third driver, high speed SAS drives.

To some extent you can say that DAS has held serve but that assumes that network storage and SAN have not evolved as well. In our next entry we will further examine the capacity amortization comparison of DAS vs. SAN and then we will look at the simplicity aspect

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George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.