Fast IO

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 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 Windows32 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 11-14.

Figure 11-14. Fast I/O decision tree

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

1. A thread performs a read or write operation.

2. 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. 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.

3. If the file system driver's fast I/O routine determines that fast I/O is possible, it calls the cache manager 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.)

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

5. 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.

6. 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.

   • 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.

Note

The fast I/O path isn't limited to occasions when the requested data already resides in physical memory. As noted in steps 5 and 6 of the preceding list, the cache manager simply accesses the virtual addresses of the already opened file where it expects the data to be. If a cache miss occurs, the memory manager dynamically pages the data into physical memory.

The performance counters or system variables listed in Table 11-10 can be used to determine the fast I/O activity on the system.

Table 11-10. System Variables for Determining Fast I/O Activity

Performance Counter (frequency)	System Variable (count)	Description
Cache: Sync Fast Reads/Sec	CcFastReadWait	Synchronous reads that were handled as fast I/O requests
Cache: Async Fast Reads/Sec	CcFastReadNoWait	Asynchronous reads that were handled as fast I/O requests (These are always zero because asynchronous fast reads aren't done in Windows.)
Cache: Fast Read Resource Misses/Sec	CcFastReadResourceMiss	Fast I/O operations that couldn't be satisfied because of resource conflicts (This situation can occur with FAT but not with NTFS.)
Cache: Fast Read Not Possibles/ Sec	CcFastReadNotPossible	Fast I/O operations that couldn't be satisfied (The file system driver decides; for example, files with byte range locks can't use fast I/O.)