#ifndef _NTIOAPI_H

typedef enum _IO_PRIORITY_HINT
{
    IoPriorityVeryLow = 0, // Defragging, content indexing and other background I/Os.
    IoPriorityLow, // Prefetching for applications.
    IoPriorityNormal, // Normal I/Os.
    IoPriorityHigh, // Used by filesystems for checkpoint I/O.
    IoPriorityCritical, // Used by memory manager. Not available for applications.
    MaxIoPriorityTypes
} IO_PRIORITY_HINT;

#endif
View code on GitHub

// wdm.h

typedef enum _IO_PRIORITY_HINT {
  IoPriorityVeryLow,
  IoPriorityLow,
  IoPriorityNormal,
  IoPriorityHigh,
  IoPriorityCritical,
  MaxIoPriorityTypes
} IO_PRIORITY_HINT;
View the official Windows Driver Kit DDI reference

NtDoc

This enumeration is documented in Windows Driver Kit.

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Windows Driver Kit DDI reference (ne-wdm-_io_priority_hint)

_IO_PRIORITY_HINT enumeration

Description

The IO_PRIORITY_HINT enumeration type specifies the priority hint for an IRP.

Constants

`IoPriorityVeryLow`

Specifies the lowest possible priority hint level. The system uses this value for background I/O operations.

`IoPriorityLow`

Specifies a low-priority hint level.

`IoPriorityNormal`

Specifies a normal-priority hint level. This value is the default setting for an IRP.

`IoPriorityHigh`

Specifies a high-priority hint level. This value is reserved for use by the system.

`IoPriorityCritical`

Specifies the highest-priority hint level. This value is reserved for use by the system.

`MaxIoPriorityTypes`

Marks the limit for priority hints. Any priority hint value must be less than MaxIoPriorityTypes.

Remarks

For more information about priority hints, see Using IRP Priority Hints.

IO_PRIORITY_HINT - NtDoc