TAILIEUCHUNG - Microsoft SQL Server 2008 R2 Unleashed- P149

Microsoft SQL Server 2008 R2 Unleashed- P149:SQL Server 2005 provided a number of significant new features and enhancements over what was available in SQL Server 2000. This is not too surprising considering there was a five-year gap between these major SQL Server 2008 is not as much of a quantum leap forward from SQL Server 2005 | 1424 CHAPTER 38 Database Design and Performance Cache Performance One of the reasons SANs can offer superior performance to locally attached storage is they typically are configured with a significant amount of cache space. This is normally a good thing. However because the SAN provides storage services to multiple servers the available cache space is shared as well. If there is significant activity against the SAN there can be extensive cache turnover. This means that the large cache space may not always be available to SQL Server so some of the performance gains provided by the large cache are not realized. NOTE Cache turnover in a SAN can lead to widely varying physical I O response times. When SQL Server performs I O against the SAN it s considered a physical I O whether or not the data resides in the SAN cache. When the physical I O performance for SQL Server is measured the performance can be orders of magnitude faster when the data is residing in the SAN cache than when the data has to be physically read from the disks in the SAN. It is important that you perform benchmarking with your SAN vendor to ensure that your SAN cache will be adequate to provide optimal database performance. Avoid Disk Drive Contention SAN storage is divided into LUNs. Servers attached to the SAN recognize one or more of these units as a disk partition or drive. However these LUNs may share the same disk drives. For example consider six 100GB drives in the SAN. Theoretically this could be divided into two LUNs of 300GB each. Although each LUN may be allocated to different SQL Servers some of the drives shared between the two LUNs could experience twice the I O from both servers than if the drives were dedicated to a single server. To avoid this situation most SANs support zoning which allows the SAN administrator to dedicate entire disks in the SAN to your LUN to isolate the I O on the drives in the LUN to your SQL Server. In addition you should try to ensure that your database log .

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