Filtering IRPs and Fast I/O
A file system filter driver filters I/O requests for one or more file systems or file system volumes. Each I/O request appears as an I/O request packet (IRP) or fast I/O request. IRPs are I/O system structures that are handled by a driver's IRP dispatch routines. Fast I/O requests are handled by the driver's fast I/O callback routines.
When a filter driver is initialized, its DriverEntry routine registers the filter driver's IRP dispatch routines and fast I/O callback routines. Only one set of these routines can be registered for each filter driver.
Some types of IRPs have fast I/O equivalents, and some fast I/O requests have IRP equivalents. However, IRPs handle many types of I/O that fast I/O cannot. Also, certain specialized fast I/O routines are used to preacquire file system resources for the Cache Manager or Memory Manager without creating an IRP. Thus, for the most part, IRPs and fast I/O requests perform separate roles in I/O operations.
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