SQL Server read-ahead mechanism; concept and performance gains

The user’s read requests in SQL Server are managed and controlled by the SQL Server Relational Engine, that is responsible for determining the most optimized access method, such as index scan or table scan, to retrieve the requested data. These read requests are also optimized internally by the SQL Server Storage Engine, the buffer manager components specifically, that is responsible for determining the general read pattern to be performed.

When you submit a query to request data in SQL Server, the SQL Server Database Engine will request that data pages that are required for your query from the buffer cache, performing a logical read. If these pages are not found in the buffer cache, a physical read will be performed to copy the pages from the disk into the buffer cache.

Although the SQL Server query optimizer tries to do its best in providing the most optimal execution plan that helps to retrieve the data requested by the user, you may still face CPU or I/O performance issues while executing the query. SQL Server provides us with many features that help in optimizing the data retrieval performance in order to respond to the user’s requests as fast as possible. One of these useful features is the read-ahead mechanism. As the name indicates, using the read-ahead mechanism, the SQL Server Storage Engine brings the data and index pages into the buffer cache, up to 64 contiguous pages per each file, before they are actually requested by the SQL Server Relational Engine, to respond for the user’s query. This provides more possibilities to find the data page in the buffer cache when it is requested and optimizes I/O performance by performing more logical reads, which is faster than physical reads. It allows also for computation overlap that helps in reducing the CPU time required to execute the queries.

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