Windows® Internals, Sixth Edition, Part 2

The DMA interface returns to the file system the physical addresses of cached user data (rather than the virtual addresses, which the mapping and pinning interfaces return), which can then be used to transfer data directly from physical memory to a network device. Although small amounts of data (1 KB to 2 KB) can use the usual buffer-based copying interfaces, for larger transfers the DMA interface can result in significant performance improvements for a network server processing file requests from remote systems. To describe these references to physical memory, a memory descriptor list (MDL) is used. (MDLs are introduced in Chapter 10.)

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 8. 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 first read or write to a file requires setting up the file for caching (mapping the file into the cache and setting up the cache data structures, as explained earlier in the section Cache Data Structures). Also, if the caller specified an asynchronous read or write, fast I/O isn’t used because the caller might be stalled during paging I/O operations required to satisfy the buffer copy to or from the system cache and thus not really providing the requested asynchronous I/O operation. But even on a synchronous I/O, the file system driver might decide that it can’t process the I/O operation by using the fast I/O mechanism, say, for example, if the file in question has a locked range of bytes (as a result of calls to the Windows LockFile and UnlockFile functions). Because the cache manager doesn’t know what parts of which files are locked, the file system driver must check the validity of the read or write, which requires generating an IRP. The decision tree for fast I/O is shown in Figure 11-11.

Figure 11-11. Fast I/O decision tree

These steps are involved in servicing a read or a write with fast I/O:

A thread performs a read or write operation.

If the file is cached and the I/O is synchronous, the request passes to the fast I/O entry point of the file system driver stack. If the file isn’t cached, the file system driver sets up the file for caching so that the next time, fast I/O can be used to satisfy a read or write request.

If the file system driver’s fast I/O routine determines that fast I/O is possible, it calls the cache manager’s read or write routine to access the file data directly in the cache. (If fast I/O isn’t possible, the file system driver returns to the I/O system, which then generates an IRP for the I/O and eventually calls the file system’s regular read routine.)

The cache manager translates the supplied file offset into a virtual address in the cache.

For reads, the cache manager copies the data from the cache into the buffer of the process requesting it; for writes, it copies the data from the buffer to the cache.

One of the following actions occurs:

For reads where FILE_FLAG_RANDOM_ACCESS wasn’t specified when the file was opened, the read-ahead information in the caller’s private cache map is updated. Read-ahead may also be queued for files for which the FO_RANDOM_ACCESS flag is not specified.

For writes, the dirty bit of any modified page in the cache is set so that the lazy writer will know to flush it to disk.

For write-through files, any modifications are flushed to disk.

Read-Ahead and Write-Behind

In this section, you’ll see how the cache manager implements reading and writing file data on behalf of file system drivers. Keep in mind that the cache manager is involved in file I/O only when a file is opened without the FILE_FLAG_NO_BUFFERING flag and then read from or written to using the Windows I/O functions (for example, using the Windows ReadFile and WriteFile functions). Mapped files don’t go through the cache manager, nor do files opened with the FILE_FLAG_NO_BUFFERING flag set.

Note