Fast I/O
Whenever possible, reads and writes to cached files are handled by a high-speed mechanism named fast I/O. Fast I/O is a means of reading or writing a cached file without going through the work of generating an IRP, as described in Chapter 9. With fast I/O, the I/O manager calls the file system driver's fast I/O routine to see whether I/O can be satisfied directly from the cache manager without generating an IRP.
Because the cache manager is architected on top of the virtual memory subsystem, file system drivers can use the cache manager to access file data simply by copying to or from pages mapped to the actual file being referenced without going through the overhead of generating an IRP.
Fast I/O doesn't always occur. For example, the ...
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