IO_PRIORITY_HINT - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#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

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This enumeration is documented in Windows Driver Kit.