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SAN and NAS underdeliver

One reason DAS continues to live on is that SAN and NAS have largely underdelivered on their promises. SANs were supposed to make it easy to create a global pool of storage that could be dynamically divvied up among servers so that only the capacity actually needed at the time was assigned to a server. For the first eight years or so of the technology’s existence, this capability was largely unavailable, and SAN storage had to be hard partitioned to individual servers. When a server needed more capacity, a new partition had to be allocated to that server and then concatenated into the existing storage pool on the server or, worse, managed separately. The process of adding storage to a server on a SAN was very similar to the prior DAS methodology.

Data protection was also supposed to get a lot easier. The goal was to back up the SAN directly and not have to back up the individual servers. While a few software applications were able to accomplish that feat, all suffered from blindly backing up data and not understanding what that data was. Users quickly realized they needed something called “

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application awareness” to back up active applications and then perform intelligent restores. As a result, some form of backup software was required on the servers.

Finally, the price of SAN or NAS technology is still significantly higher than DAS. Many users have decided it’s less expensive to inefficiently directly attach storage than to efficiently share it.

To be fair, modern SAN and NAS implementations have addressed the early storage allocation shortcomings with technologies like thin provisioning. However, the time it took to deliver on the allocation promise allowed DAS to build on its foothold in the data center. But the other challenges remain, for the most part.

The primary driver for SAN/NAS adoption has been the advent of server and desktop virtualization, since the ability to move virtual server images between physical hosts requires shared storage. Virtualization also makes application-aware, off-host backup viable due to the entire server being a file that can be backed up without interacting with the original physical host. But despite this new and important use case for shared storage, DAS continues to live on in the data center. And its value is increasing.

This was first published in May 2012

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