Moving files causes time machine to backup

Question

Why does moving a file or files necessitate TM backing them up again? Is it not a bit more intelligent than that? After doing a bit of house keeping TM decides to do a huge backup just because I have moved some files, ones which are already backed up, is this just the way it is or is there more to it?
cheers...rob

Answer 1

Well, actually they are in a sense. The directory entries are being rewritten although the data itself are not moved. But then when the backup utility runs and scans the drive it no longer finds those moved files. It will then delete them from the backup. When it finds them in their new location it copies them from the source location to the new destination location (making any new folders if required.)
Very few backup utilities (and TM is not one of them) have implemented a sort of fast scan to reduce the scan time required for a standard backup. The implementation requires that the user not change the source or destination devices because the backup utility only scans the source for changes then writes those changes to the destination. This method substantially reduces the time for a backup especially if you backup on, say, a daily basis. The only ones I have found are Synk and Synchronize! Pro X. Now, Retrospect is different in that it creates an index file when doing a backup. Subsequent backups are then quicker because it only has to compare the source to the index, and searching the index is very fast compare to scanning a volume. But Retrospect is a pretty heavyweight backup program and can be a bit difficult to master.