Windows Vista changes the behavior of the file copying operations, by using non-cached I/O requests (Before it was cached). This means that it will not check if the file in the system file cache before starting the file copy, and the file will not enter the cache during the direct file copy from source to destination.

• Update with Vista SP1 - The default file copy routine has been reverted back to perform cached file copying by default. Except when a remote file is read or written, then it tells the client-side remote file system driver (Rdbss.sys) not to perform caching, so only the local read or write operation is cached in memory. More Info Mark Russinovich

Normally when copying large files using cached I/O request, then it can ruin the file cache by either flushing out all other cached files, or make it grow so large that it depletes the physical memory or unloads memory to the pagefile. Therefore it is a good idea to use non-cached I/O when doing file copying.

But apparently the I/O rutines in the system file cache manager are so fast that even if it ruins the file cache, then the cached file copying is quicker. This is probably because the file cache works like a large read ahead buffer. When doing a file copy on the same disk, then it doesn't have to constantly move the hard disk head back and forth between source and destination. Another reason why cached file copying feels quicker is that when the cache manager has read the entire file into memory, then the file copy dialog is closed before it has finished writing to the destination file.

Configure this DWORD registry value to use cached I/O requests when doing file copy:

[HKEY_LOCAL_MACHINE \SOFTWARE \Policies \Microsoft \Windows \System]
CopyFileBufferedSynchronousIo = 1 (Default = 0)

More Info MS KB941673 (Hotfix is required)

Note Microsoft have identified several performance issues in Windows Vista, and created several patches that address some of these issues. MS KB938194, MS KB938979, MS KB941649, MS KB941600, MS KB943899 (Will over time be available on Windows Update, and will be part of Vista SP1).

Related Slow file copy caused by automatic tuning of TCP/IP RWIN

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