Vista SP1 Fixes Slow File Copies--Here's Why

One of the first things I noticed when setting up Vista for the first time was that file copies seemed to be slower. Plenty of other people noticed it, too. This week, Mark Russinovich put together a blog entry with details about how this Vista "improvement" went awry.This is to some extent a story about how no good deed goes unpunished. Windows XP has its own bad behavior when it comes to file copies. Copying large files will cause other useful code and data to be thrown out of memory, and that can cause poor performance and swapping down the line. So Microsoft set out to fix this problem with Vista. Many changes were made; one of the most important was to avoid using the file cache on file copies larger than 256 KB, and instead do the file I/O directly to disk.

In some cases, this change to noncached I/O made file copies slower. One of the situations Russinovich identifies is when there are real-time virus and/or spyware scanners in use. Reading or writing any file often triggers a flurry of activity by these programs; noncached I/O creates a further bottleneck as these programs vie to take a look at the copied bytes.

Other times, Vista's reduced performance was more perceived than real. When the I/O is cached, as it is on XP, the copy dialog may go away before the last of the file is actually written to disk. In Vista, the copy dialog stays up until the last byte is written, which gives the user the impression that the copy operation was slower.

Starting with Vista SP1, Microsoft has returned to using cached I/O for file copies when the files are on the local system. There are some downsides to this in certain situations, as Russinovich details in his blog post, but on balance it will probably give users the copy performance they have come to expect on XP systems.

I would love to see Microsoft provide this kind of detail more often about bugs and fixes. It is easier to have more sympathy for Microsoft's design choices when you can see the reasoning behind changes that initially seem to create more hassles than they fix.