SQL Server 2000 I/O Configuration in a SAN/NAS Environment


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. This is illustrated here.

A 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. So, in a NAS environment, in may not be possible to achieve 10ms I/O accesses, depending on the NAS. This might adversely affect your performance.


The basic performance problems and solutions of running SQL Server in a SAN/NAS environment are related to the I/O capacity of the disk drives, the fibre channel network, the LAN, and the software overhead. Rigorous tuning should be done on a regular basis in order to make sure that the capacity of the I/O subsystem is not overloaded. In the case where the I/O subsystem is overloaded, you may need to add disk drives, change RAID levels, add RAID controllers or a combination of all of these. SQL Server performance is all about I/O performance. I/O performance is mainly affected by RAID level and by the number of disk drives and the rate the I/O subsystem is driven at. An overloaded disk subsystem can cause severe performance problems. 

About the Author

Edward Whalen is vice president and principle consultant at Performance Tuning Corporation (http://www.perftuning.com/). Performance Tuning Corporation provides database performance tuning, load testing and troubleshooting services on MS SQL Server. Edward Whalen was a co-author on four SQL Server books from Microsoft Press; SQL Server 7 Administrator’s Companion, SQL Server 7 Performance Tuning Technical Reference, SQL Server 2000 Administrator’s Companion and SQL Server 2000 Performance Tuning Technical Reference. Edward Whalen has also authored four Oracle books. Edward Whalen is considered a leader in database performance tuning.

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