docs about ZwReadFileScatter and ZwWriteFileGather

Where can I find descriptions about ZwReadFileScatter and ZwWriteFileGather?
The question is, what are the parameters for 'ULONG Length'?
I would write into a file from scattered buffers with one APC callback
at end of all buffers.

Have nothing found about on msdn. Only I found backtracers from craches.
And this header: http://www.cs.colorado.edu/~main/cs1300/include/ddk/ntapi.h

NTOSAPI
NTSTATUS
NTAPI
ZwReadFileScatter(
IN HANDLE FileHandle,
IN HANDLE Event OPTIONAL,
IN PIO_APC_ROUTINE ApcRoutine OPTIONAL,
IN PVOID ApcContext OPTIONAL,
OUT PIO_STATUS_BLOCK IoStatusBlock,
IN PFILE_SEGMENT_ELEMENT Buffer,
IN ULONG Length,
IN PLARGE_INTEGER ByteOffset OPTIONAL,
IN PULONG Key OPTIONAL);

Henry Nestler
.

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