Kernel space: Once upon atime
Among the metadata maintained by most filesystems is the last file access time, or "atime." This time can be a useful value to have - it lets an administrator (or a program) know when a file was last used. There is a strong downside to this feature, though: it forces a write to the disk every time a file is accessed. So read-only operations, which might have been satisfied entirely from cache, turn into filesystem writes to keep the atime value up to date.
A recent discussion on write throttling turned to atime after Ingo Molnar pointed out that atime was probably a bigger performance problem than just about everything else. He went on to say:
Atime updates are by far the biggest IO performance deficiency that Linux has today.