DAS
DAS is the traditional or legacy type of storage. This type of storage is non-networked—that is, it’s connected directly to a server. The main advantages of DAS are that it’s easy to install and manage, and it’s inexpensive. However, because DAS isn’t shared between computers, its use is limited to individual machines. Therefore, when you need to increase storage capacity, you often end up with multiple (albeit inexpensive) hard drives. With DAS, you also run the risk of running out of storage capacity. DAS is also difficult and time consuming to upgrade or expand. NAS and SANs address some of these limitations.

NAS
NAS is shared storage connected to the network. NAS appliances are simple to install. Initially, NAS is more expensive than DAS (but less expensive than a SAN). However, centralized storage makes NAS cheaper and easier to administer overall than DAS. NAS provides high storage capacity, easy data sharing, resource consolidation, and quick file access for multiple clients. However, NAS is less efficient than a SAN for moving large chunks of data, and it doesn’t provide as many configuration options. In addition, NAS isn’t entirely suitable for SQL Server. Although you can use NAS for SQL Server database storage, few organizations do so because NAS often doesn’t provide the same level of performance as DAS. NAS is a shared storage solution, which means that I/O is limited by the network connection.

SAN
A SAN is separate network specifically for attaching storage devices. SANs are very reliable, scalable, and fault tolerant. In addition, SANs provide better availability than NAS because all the storage devices are available on all the servers. End users benefit from the optimized network capacity and maximum utilization of server power. SANs are extremely efficient at moving large chunks of data. They’re most applicable to large databases or to bandwidth-intensive or mission-critical applications. However, SANs are complex to manage and are very expensive. Early implementations of SANs required Fibre Channel connections, which added to the complexity and cost. (For more information about SANs, see "What’s the Best Way to Carve Up a SAN?" September 2007 and "SQL Server on a SAN," January 2006.)

iSCSI SANs
With the emergence of iSCSI SAN technology, the cost of implementing a SAN greatly decreased. iSCSI SANs use a standard Ethernet infrastructure to transmit data between devices. Although iSCSI SANs offer greater flexibility in remote storage than traditional Fibre Channel SANs, they're slower. In addition, iSCSI SANs aren’t any easier to manage than Fibre Channel SANs are. For more information about iSCSI SANs, check out the Windows IT Pro Buyer’s Guide "iSCSI SANs for SMBs."