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Article ID: 160606 - Last Review: October 26, 2013 - Revision: 2.0

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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(
<WWBLOCKQUOTE>
    HANDLE hFile,                         // handle of file to read
    FILE_SEGMENT_ELEMENT aSegmentArray[], // array of buffer segments that
receives data
    DWORD nNumberOfBytesToRead,           // number of bytes to read
    LPDWORD lpReserved,                   // reserved must be NULL
    LPOVERLAPPED lpOverlapped             // address of structure for data
   );
				

Parameters

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.

aSegmentArray

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.

NNumberOfBytesToRead

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

lpReserved

Reserved must be NULL.

lpOverlapped

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

The read operation starts at the offset specified in the OVERLAPPED structure and ReadFileScatter may return before the read operation has been completed. In this case, ReadFileScatter returns FALSE and the GetLastError function returns ERROR_IO_PENDING. This allows the calling process to continue while the read operation finishes. The application can use either GetOverlappedResult/HasOverlappedIoCompleted to pick up I/O completion notification, or GetQueuedCompletionStatus to pick up I/O completion.

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
containing the data
    DWORD nNumberOfBytesToWrite,    // number of bytes to write
    LPDWORD lpReserved,             // Reserved must be NULL
    LPOVERLAPPED lpOverlapped       // pointer to structure needed for
overlapped I/O
   );
				

Parameters

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.

aSegmentArray

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.

NNumberOfBytesToWrite

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.

lpReserved

Reserved must be NULL.

LpOverlapped

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.

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.

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

Windows NT 4.0 Service Pack 2 introduced a new interface that acts as a performance enhancer for SQL Server when running under Windows NT. This enhancement involves scatter/gather and input/output functionality.

For additional information on obtaining the latest service pack for Windows NT 4.0, click the article number below to view the article in the Microsoft Knowledge Base:
152734  (http://support.microsoft.com/kb/152734/EN-US/ ) How to Obtain the Latest Windows NT 4.0 Service Pack

Applies to
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
Keywords: 
kbnosurvey kbarchive kbnetwork KB160606
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