One of the peculiar features of Microsoft SQL Server is its capability to keep alive multiple threads at the same time for performing I/O operations. The configuration options that affects how many threads available for these operations is max async IO. The default value for this option is 32, which is adequate for most cases. However, when the server computer can leverage advanced sub-systems for managing disk arrays, this value can and should be increased because more threads can work at the same without affecting negatively disk performance. On the other hand, it is important not to use too large a value, which would increase lazy writes and checkpoints and decrease the number of read requests.

There are several strategies to fine-tune the performance in this case. One of the simplest techniques is to increase the option value and use the system Performance Monitor to check the most significant SQL Server counters, such as the “Batch requests/sec” of the “SQLServer:SQL Statistics”. If the option setting is optimal, this counter should maintain a large value. If this isn’t the case, it is evident that write operations tend to exhaust the I/O capabilities of the disk sub-system. For more information about advanced performance tuning techniques, see Chapter 15 of “Inside Microsoft SQL Server 7”, by Ron Soukup and Kalen Delaney (Microsoft Press).########################################################

This tip has been originally published on Microsoft Italia’s web site.
It has been translated and re-published on VB2TheMax with the permission of Microsoft Italia.
You can find more tips like this one (in Italian) at http://www.microsoft.com/italy/sql/articoli

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