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SET_DAX_ALLOC_ALIGNMENT_HINT_INPUT structure (ntifs.h)

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After Windows issues the SET_DAX_ALLOC_ALIGNMENT_HINT_INPUT file system control for a file, it examines allocations to the file.

Once this file system control is issued for a file, the following allocations to the file will be examined to see if the given offset is being allocated. If so, the allocation's physical address will be aligned to the given AlignmentValue if enough free space is available at the aligned regions.

Syntax

typedef struct _SET_DAX_ALLOC_ALIGNMENT_HINT_INPUT {
  ULONG     Flags;
  ULONG     AlignmentShift;
  ULONGLONG FileOffsetToAlign;
  ULONG     FallbackAlignmentShift;
} SET_DAX_ALLOC_ALIGNMENT_HINT_INPUT, *PSET_DAX_ALLOC_ALIGNMENT_HINT_INPUT;

Members

Flags

The flags used for the file system control. There are two flags that can be used:

Value Meaning
DAX_ALLOC_ALIGNMENT_FLAG_MANDATORY If allocation satisfying AlignmentShift (or at least FallbackAlignmentShift if specified) can't be found, then fail the file system operation (e.g. extending the file).
DAX_ALLOC_ALIGNMENT_FLAG_FALLBACK_SPECIFIED Indicates that the FallbackAlignmentShift field is present in the input structure and indicates a fallback alignment if the optimal alignment isn't available.

AlignmentShift

The block size used to align the given offset of the file. The file is allocated so that the file's physical address is aligned to this block size.

The actual alignment value is given by:

AlignmentValue = (1 << AlignmentShift);

FileOffsetToAlign

The file offset that should be aligned to the given AlignmentValue. This should be cluster aligned.

The Memory Manager supports huge page only if the file offset and the physical address are 1gb aligned.

FallbackAlignmentShift

If DAX_ALLOC_ALIGNMENT_FLAG_FALLBACK_SPECIFIED is present in Flags, this field specifies a fallback block size to align the given offset of the file whenever allocation satisfying AlignmentShift could not be found.

Remarks

To reset the alignment constraint you can issue this FSCTL with an AlignmentValue of 1; that is, AlignmentShift as 0.

Requirements

Requirement Value
Minimum supported client Windows 10, version 1709
Header ntifs.h (include Ntifs.h)