However, as the volume of data and the number of users grows, DAS becomes harder to manage. That is when companies start looking at the next step in storage. While some of those companies go directly to a SAN, the most common next step is to add storage to the existing network in the form of NAS.
When to move
The reason to move from DAS to NAS is economics. NAS is the easiest and least expensive way to add storage to a network, so most SMBs that are moving away from DAS start with NAS. This is often done in the form of inexpensive NAS filers, which combine disk storage and network connectivity in a single box, along with enough smarts to manage everything.
In general, the more network-centric your operation, the more you need to consider NAS or a SAN. Likewise, as users start to bump against the capacity of their DAS, they should consider whether to add storage to a bunch of systems or add a single, more efficiently utilized chunk of storage to the network. NAS, even with simple NAS appliances, costs more per gigabyte than adding a new disk drive. But the higher utilization and ease of allocation can easily cover the added cost.
One of the biggest advantages of low-end NAS is simplicity. Most NAS vendors, like NetApp, offer inexpensive NAS appliances that can be brought up on a network in less than an hour by staff with minimal training.
NAS appliances range in capacity from a couple of hundred gigabytes to several terabytes of storage and in price from approximately $1,000 to $50,000 or even more. Besides higher capacity, the more expensive units feature more sophisticated management and data backup tools.
The physical architecture of basic NAS is very simple. The logical architecture -- what to move to NAS and what to keep in DAS -- requires a little more thought.
The ideal files for NAS are reasonably sized, commonly used by multiple users (especially in the same department or office) and do not have very high-performance requirements. While some companies have moved transaction processing applications onto NAS appliances, that usually isn't considered ideal because of the potential for network congestion.
It is important to note that thin provisioning (TP) on NAS works fine if your network isn't too heavily loaded. However, if you've got an active TP application, your network is likely to be the first bottleneck.
Before installing a NAS system, spend some time understanding your users' requirements. Determine the files and applications that are most often shared, as well as which DAS systems are running low on storage. Also look for duplicate DAS files and folders that can be eliminated by putting a single copy on NAS. Such duplication is a common response to the performance problems of using a file on someone else's computer over the network. The remote user copies the file over to get more speed.
Getting a file from a NAS box over the network is not going to be as fast as getting it from DAS on a user's computer. But it should be considerably faster than getting it from another user's computer and perhaps faster than getting it off the server.
The most common problem with adding NAS filers is the extra load they put on the network. Because the files will be traveling back and forth over your network, NAS has the potential to cause network congestion, especially if your network is already heavily loaded.
This is less of a problem now that NAS appliances routinely come with built-in Gigabit Ethernet connections, but you may have to upgrade your network to support NAS and keep adequate performance. Remember that all the applications on the network are likely to be affected by the added NAS load.
Ideally, you want your NAS filer to be in close topological proximity to the servers and workstations that are using those files. The more hops through gateways you have to make to get from the filer to the destination, the more performance will suffer.
NAS and storage management
Because NAS filers are so easy to add, there's a tendency to go overboard with them. That is, companies keep adding them as departments need more storage without a lot of thought to the overall storage architecture.
Surprises aside, the big problem is trying to manage a sprawling collection of NAS filers that have popped up over the network. NAS filers typically come with storage management software, but products with low-end NAS filers tend to be pretty basic and may not integrate well with your company's own storage management software.
Fortunately storage management software has become more NAS-aware and it is easier to manage NAS filers on modern networks. However, it is important that your company has policies in place to manage the proliferation of NAS filers.
At some point it makes sense to go to more sophisticated, more manageable NAS systems, such as NAS gateways, or to go to a SAN for most of your storage needs.
The final step in moving from DAS to NAS is migrating everything to its new home. Moving the data itself is straightforward. You can copy the files directly or restore them from a backup onto the NAS appliance. The more difficult part of the job is making sure everyone can find their files in their new locations.
In Windows, the basic method is to substitute the name of the new platform for the old server in the Group Policy logon scripts. However, this won't work if a user is accessing the file through something like dragging a shortcut of a folder onto their desktop. In this case, DNS renaming might be the appropriate approach.
About the author: Rick Cook specializes in writing about issues related to storage and storage management.
This was first published in July 2008