#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;
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This enumeration is documented in Windows Driver Kit.