Backed up data from Time Machine

Question

Hi,
I just installed Leopard on my iBook G4 and for Time Machine I used my 250 GB external HD from iomega, which is very good. I always thought that at the beginning Time Machine copies everything on the external HD, and then every hour it just checks what has been changed and copies only the changed files over. I just browsed through my external HD and saw that, as a matter of fact, Time Machine backs up every file every hour. I noticed this because in the "Backup" folder (on the ext. HD) there were many other folders with time and date. You could see that for every hour Time Machine made a folder, with all my documents inside. Is this right? Does Time Machine really back up the whole computer every hour? Or did I just see something wrong? Because if Time Machine really backs up 30 GB of data every hour, I have a slight problem with a 250 GB external HD.
Thanks!

Answer 1

Open up System Preferences / Time Machine.
Now right-click (or CTRL-click) on the Time Machine icon in the Dock and select Back Up Now from the menu that pops up. Watch what happens in the Time Machine preferences pane. That will show you how much is being transferred. You can also watch when the time for the next hourly backup approaches.
You can also see this info after the fact by going to Applications / Utilities / Console (select "All Messages" in the column on the left) and looking for the information messages put out by the "backupd" process that is actually doing the work.
What you'll find is that far less than the whole system gets transferred. However, despite that, each Time Machine snapshot in time includes the COMPLETE file system? How does it do it? By including "hard links" -- a special form of file pointer or alias -- for each file or folder that was actually transferred in an earlier incremental or initial backup. When Time Machine discards old backups (to winnow hourly backups down to one backup per day, and eventually one backup per week), the file system doesn't actually delete a file on the Time Machine disk unless all the hard links to it are also gone. So if a file was transferred in a given incremental backup, it will STAY on the Time Machine drive until all subsequent "hard link" references to it are deleted -- even though the backup during which it was originally transferred has long since been deleted itself.
So if you delete the real file on your computer, no new hard links get created as part of each subsequent Time Machine backup, and eventually (as the disk gets full and old backups are discarded) the last reference to that file will be discarded and the file itself will also actually be deleted from the Time Machine drive. And if you change a file, the new version gets transferred, but the old version remains on the Time Machine drive so long as there are still hard link references to it which have not yet been discarded.
From a user point of view, you don't have to know what got transferred during a given incremental backup, because each and every time window into your Time Machine always shows you the COMPLETE file system as of that point in time (minus anything you have put into the Do Not Back Up list).
--Bob