SQL Server Performance in a SAN/NAS Environment

SAN and NAS Systems

SAN and NAS systems provide different types of storage from traditional internal or direct attached storage. A SAN (Storage Area Network) is designed to provide access to storage over a private fibre channel network. A NAS (Network Attached Storage) is used to provide storage over a standard network.

SAN SYSTEMS

A SAN system is an external storage system that allows multiple computer systems to access the same storage. The RAID controller inside the external storage system is able to take requests for different logical volumes within the storage system from different HBAs (Host Bus Adapters). This allows for several different features. One of the most common uses of a SAN is for storage consolidation. This is where multiple systems share the disks in the external storage subsystem. This allows for consolidation of storage resources and management.

With storage consolidation, even though the storage in the external disk subsystem is shared among the different systems, it is not entirely accessible to all systems. Logical disks are carved out of the physical disk drives and allocated to each of the computer systems. Only one system can access a particular disk volume.

Another use for a SAN system is for clustering. Failover clusters use a shared disk subsystem that allows one of two systems to access the same storage.

SAN Performance Considerations When putting together a SAN system you must not only look at the I/O traffic that is being generated by one system, but the I/O traffic being generated by all systems in the SAN. So in addition to sizing the I/O subsystem disk drives and RAID levels, you must look at the traffic on the SAN itself, as well as keeping in mind that other systems might be accessing the same RAID controllers. It is also possible to run into bandwidth limitations on the SAN itself, since fibre channel has a limited bandwidth.

NETWORK ATTACHED STORAGE (NAS) SYSTEMS

Network attached storage is similar to the SAN system in that the brains of the storage is external to the computer system. However, unlike the SAN system where the storage is connected via a fibre channel connection, a NAS system is accessed via the network.

NAS System
Although the NAS system is supported under SQL Server, the NAS system usually cannot support the performance required by SQL Server unless you use a sufficiently fast network interface. The speed of the NAS is usually limited by the speed of the network interface.

NAS Performance Considerations The NAS is different from the SAN in that multiple systems can access the same NAS storage simultaneously, whereas in a SAN, typically only one system accesses a particular file system at a time. So, the NAS may have performance problems related to too many people accessing the storage at the same time.

However, the main issue with NAS is that the path that the I/O must take typically is much longer than that of a SAN. This is due to the fact that the NAS has network protocols involved that must be processes as well as file system overhead.