Question: Why do people purchase SAN over DAS?
Answer: This is actually a question that I get a lot when talking to people about moving from DAS to a SAN platform. The short answer is manageability and better utilization of disk resources. What that really means is that when you use a SAN overall manageability of the storage environment becomes much easier as you can manage the entire storage environment from a single console. This allows for not only centralized management but monitoring as well. With a SAN platform you can see quickly and easily where your storage performance problems are which allows you to easily move resources from one server to another to increase performance where needed.
Related: SANs: Always Better Than DAS?
You get better utilization of disk resources with SAN than you do with DAS because when working with SAN the spindles aren't dedicated to a specific server. With DAS what ever server the disks are plugged into is the server that those spindles are going to be used for. If you needed 1TB of space, but had 3TB of space attached to the server, you can't allocate that 2TB of space to other servers. With a SAN platform you take the 3TB of space and carve up the space and allocate it to servers as needed. This makes adding more space to servers much easier as well. Most DAS platforms don't give you an easy way to add disk space to a single volume (or drive letter). With DAS when you need to add more disk space you need to create another disk and configure the application (or database in our case) to use that new disk. In a SAN platform you can simply tell the SAN to add additional space to the end of the disk. Then tell Windows that the disk is bigger and Windows will see the disk is bigger and the new space will be available.
Another utilization performance bonus that you get with SAN over DAS is that if you have applications which only use the disks at night and other applications which only use the disks during the day you can put these on the same physical disks for maximum utilization. This will allow you to have all the performance that you need for both applications without having to purchase double the disks. With DAS the only real way to do this would be to host both of those applications on the same physical server which would work, except that you no longer have any maintenance window for that server.
Beyond these two huge points there are a number of other things that SAN platforms can do that DAS simply can't. The most popular of these that people use is the ability to take snapshots and/or clones of LUNs. By using snapshots you can easily do things like offload the bulk of your DBCC CHECKDB commands to another server. You can also minimize the amount of space needed for down-level environments like Dev, QA and Staging which may need full data sets, but you don't have the budget for a full sized Dev, QA and staging platforms.
Assume for a moment that you have a 1TB database that you need to duplicate in your Dev, QA and Staging environments and you need to be able to refresh these databases on a monthly bases from production. With DAS you would need to backup and restore the 1TB database to each of the three other platforms. This requires purchasing 4TB of space, 1TB of space for each environment. By using snapshots on a SAN instead you could snapshot the production LUNs and then present these snapshots to the Dev, QA and Staging platforms. At this point the only space used by the Dev, QA and Staging platforms will only be what ever data is changed in these platforms. Typically this would only be a small amount of data. On a 1TB database the amount of data which changes in Dev, QA and Staging would probably be around 10 Gigs per month. So added up the four platforms would now only require about 1.03TB of space (1TB for Production and about 10 Gigs for each of the other environments).
Yet another benefit that SAN storage gives you over DAS is the ability to quickly and easily change the RAID level which is used under the LUN. In a DAS to change the RAID level you'll need to delete the disk, rebuild the RAID array with the new RAID level then restore the data. On a SAN platform you can simple configure other disks in the needed RAID level then move the LUN from the old RAID set to the new RAID set. With DAS this would be a multi-hour down time to do this. With a SAN platform this can all be done with no outage to the system, and only a minimal performance hit to the performance of the system while the data is being moved to the new RAID set. (This ability will depend on your SAN platform as some SAN platforms require that you use only a specific RAID level instead of being able to select the RAID level that you want to use, so check with your SAN vendor to see if you have this option or not.)
Hopefully this helps answer why you have, or may want to have a SAN platform in your environment.