Performance Enhancements for SQL Server Under Windows NT

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0

SUMMARY

A new interface was added as a performance enhancement for SQL Server when running under Windows NT. This enhancement involves scatter/gather I/O functionality.

MORE INFORMATION

What is Scatter/Gather?

Scatter/gather is a standard technique used in some high-performance I/O situations. It typically refers to Direct Memory Access (DMA) transfers. A given block of data might exist in memory as several discrete buffers. Without scatter/gather, when setting up a memory-to-disk transfer, the operating system will often do a transfer for each buffer, or block move them to a single larger buffer before initiating the transfer. Both of these are inefficient. It is better if the operating system/driver/hardware gathers up the data from the discrete locations in memory and transfers them "in place" as a single operation. Likewise, on input from disk, if the data block is scattered directly to the necessary locations in memory without intermediate block moves, and so on, it is faster.

Interface Specification

ReadFileScatter

The ReadFileScatter function reads data from a file, starting at the position indicated by the file offset in the OVERLAPPED structure, and distributes the data into multiple user buffers.

BOOL ReadFileScatter(

    HANDLE hFile,                         // handle of file to read
    FILE_SEGMENT_ELEMENT aSegmentArray[], // array of buffer segments that

    DWORD nNumberOfBytesToRead,           // number of bytes to read
    LPDWORD lpReserved,                   // reserved must be NULL
    LPOVERLAPPED lpOverlapped             // address of structure for data
   );

hFile

   Identifies the file to be read. The file handle must have been created
   with GENERIC_READ access to the file.

   The file must be opened using FILE_FLAG_OVERLAPPED to specify
   asynchronous I/O, and FILE_FLAG_NO_BUFFERING to specify non-cached I/O.

   Points an array of pointers to segments. A segment is a memory buffer
   where part of the transferred data should be placed. Segments are system
   page sized and must be aligned on a page size boundary.

   The memory segment can be allocated using VirtualAllocVlm, or can be
   normal 32-Bit address memory but not both.

   Specifies the number of bytes to be read from the file.

   Reserved must be NULL.

   Points to an OVERLAPPED structure. This structure is required and must
   point to a valid OVERLAPPED structure.

Return Values

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero. To get extended error information, call GetLastError. If ReadFileScatter attempts to read past the end of the file, the function returns zero, and GetLastError returns ERROR_HANDLE_EOF

Remarks

Each FILE_SEGMENT_ELEMENT is 64 bits long. All 64 bits are used to calculate the buffer pointer; therefore, applications must explicitly zero the upper 32 bits of the element on systems which do not support 64-bit pointers.

WriteFileGather

The WriteFileGather function gathers data from multiple buffers and writes the data to a file. The function is designed for asynchronous operation. The function starts writing data to the file at the position specified by the OVERLAPPED structure.

BOOL WriteFileGather(

    HANDLE hFile,                // handle to file to write to
    FILE_SEGMENT_ELEMENT aSegmentArray [], // array of buffer segments

    DWORD nNumberOfBytesToWrite,    // number of bytes to write
    LPDWORD lpReserved,             // Reserved must be NULL
    LPOVERLAPPED lpOverlapped       // pointer to structure needed for

   );

hFile

   Identifies the file to be written to. The file handle must have been
   created with GENERIC_WRITE access to the file. The file must be opened
   using FILE_FLAG_OVERLAPPED to specify asynchronous I/O, and
   FILE_FLAG_NO_BUFFERING to specify non-cached I/O.

   Points an array of pointers to segments. A segment is a memory buffer
   where part of the transferred data should be placed. Segments are system
   page sized and must be aligned on a page size boundary. The memory
   segment can be allocated using VirtualAllocVlm, or can be normal 32-bit
   address memory, but not both.

   Specifies the number of bytes to write to the file.

   Unlike the MS-DOS operating system, Windows NT interprets a value of
   zero as specifying a null write operation. A null write operation does
   not write any bytes but does cause the time stamp to change.

   Reserved must be NULL.

   Points to an OVERLAPPED structure. This structure is required and must
   point to a valid OVERLAPPED structure. The write operation starts at the
   offset specified in the OVERLAPPED structure and WriteFileGather may
   return before the write operation has been completed. In this case,
   WriteFileGather returns FALSE and the GetLastError function returns
   ERROR_IO_PENDING. This allows the calling process to continue processing
   while the write operation is being completed. The application can use
   either GetOverlappedResult/HasOverlappedIoCompleted to pick up I/O
   completion notification, or GetQueuedCompletionStatus to pick up I/O
   completion.

If the function succeeds, the return value is nonzero. If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

Each FILE_SEGMENT_ELEMENT is 64 bits long. All 64 bits are used to calculate the buffer pointer; therefore, applications must explicitly zero the upper 32 bits of the element on systems which do not support 64-bit pointers.

If part of the file is locked by another process and the write operation overlaps the locked portion, this function fails.

Unlike the MS-DOS operating system, Windows NT interprets zero bytes to write as specifying a null write operation and WriteFileGather does not truncate or extend the file. To truncate or extend a file, use the SetEndOfFile function.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This problem was corrected in the latest US Service Pack for Windows NT. For information on obtaining this update, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K