'Unnecessary' Defrag.

I haven't run the Degfragmenter for a couple of months. I have done few scans in that time but the message came back that defragging was not needed at that time. Today though I thought I should run the process anyway.
Three hours or so later, the 'unnecessary' defragmentation was complete.

An unnecessary scan might have taken 15-30 minutes, I thought. At most an hour. So at what point does the OS decide that defragging is required?

Each defragmenter uses it's own algorithms to determine whether a defrag is required or not. The windows XP defragger is quite an old and basic version dating from around 2001 or so, and to put it mildly, not very 'intelligent'. If you notice that opening files takes longer, audio/video files stutter, the drive 'seeks' too much (you can hear the noise or watch the LED indicator) and there is a general decrease in responsiveness, then defragging is overdue. Ofcourse, a number of factors can contribute to a slow PC, but fragmentation is certainly one of them.

These days, automatic defragmentation is becoming popular (like in Vista, but implemented far better) where the software analyzes the drives quietly in the background and defragments automatically (if necessary) when there are free system resources to do so without disturbing the user. Since everything is automatic, it's far easier than the user running analysis and defragging periodically manually/on a schedule, especially for multiple drive setups. Usually, after one major defragmentation run when you first install the software, defragmentation occurs daily for a few minutes (< 5 min for a normal PC) to clean up that day's fragments; the user does not have to bother about anything save for the occasional boot-time defrag.

Aha. Thanks for the info.

Update - on this thread here
Whats the best Defragmentation Software?
I found a link to the Auslogics free defrag:
http://www.download.com/Auslogics-Di...html?tag=lst-3

I gave it the program a test run today and the results were quite interesting. Even though I had defragged just a week or so ago Auslogics still ran for about an hour and found a goodly number of files to defragment.