Depends on the drive technology. For SSDs you want to avoid extra writes as those wear down the cells. For traditional hard drives you can consolidate empty s

Depends on the drive technology. For SSDs you want to avoid extra writes as those wear down the cells. For traditional hard drives you can consolidate empty space to reduce cluster waste, or go for broke and just place everything in order (most fun to watch). Probably the best way would be to group frequently accessed files near each other so the heads traveled less distance. So far as the algorithm, what I observed is the program would look at how much room was needed to place the next file, then clear that many blocks out of the way by moving them to the end of the drive. Then copy the blocks for the file to form a contiguous stretch of blocks. And repeat. You wanted to have as much free space as the largest file (plus a little), but I think some of them were able to move large files in pieces. Back in the days of Windows NT we had a network share with 8+ million files on it (a lot for the time) and we had a serious fragmentation problem, where it could take a second or more to read a file. Most of the defraggers just gave up, or weren't making any progress, but we eventually found one that worked (PerfectDisk, maybe?)