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Guru's Guide to SQL Server Architecture and Internals, The by Ken Henderson

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SQL Server I/O

When running on a member of the Windows NT family, SQL Server performs as much file I/O as possible asynchronously. You'll recall from Chapter 5 that the standard Win32 API functions can be executed asynchronously when the file object they're working with has been opened with the FILE_FLAG_OVERLAPPED switch and a pointer to a valid OVERLAPPED structure is passed in to the functions that require it. For example, if you call CreateFile and pass in FILE_FLAG_OVERLAPPED, then call ReadFileEx with the returned file handle and a pointer to a valid OVERLAPPED structure, Windows will attempt to process your request asynchronously. SQL Server takes advantage of this Windows' facility to avoid blocking on I/O whenever possible.

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