Time Machine: how does it work?

[franz1999]

So I am looking at the Backups.backupdb folder.
Inside there are already a few backup folders (named by date/time) and one folder called Latest which is just an alias for the most recent of the other folders.
What puzzles me a little is that if "Get Info" of any of those folders they ALL return 108GB. But also the parent folder Backups.backupdb folder returns 108GB 😕
It feels like anyone of these folders is "virtual" in a way but at the same time I can dig in the folder for yesterday's backup and drag a file out on the desktop and, sure enough, I have the actual file saved on my desktop... 😕
How does it work? Is there really a DB file somewhere and when I browse a folder I am in reality just seeing something that doesn't exist on the TimeMachine hard drive?

 

[robbieduncan]

Unix hard links.

 

[franz1999]

Unix hard links.

Interesting... coming from a Windows background I had never heard of hard links before... so here comes for those interested a quick explanation of this concept:

A hard link is essentially a label or name assigned to a file. Conventionally, we think of a file as consisting of a set of information that has a single name. However, it is possible to create a number of different names that all refer to the same contents. Commands executed upon any of these different names will then operate upon the same file contents.

To make a hard link to a pre-existing file, enter:

ln oldfile newlink

Replace oldfile with the original filename, and newlink with the additional name you'd like to use to refer to the original file.

This will create a new item in your working directory, newlink, which is linked to the contents of oldfile. The new link will show up along with the rest of your filenames when you list them using the ls command. This new link is not a separate copy of the old file, but rather a different name for exactly the same file contents as the old file. Consequently, any changes you make to oldfile will be visible in newlink.

You can use the standard Unix rm command to delete a link. After a link has been removed, the file contents will still exist as long as there is one name referencing the file. Thus, if you use the rm command on a filename, and a separate link exists to the same file contents, you have not really deleted the file; you can still access it through the other link. Consequently, hard links can make it difficult to keep track of files.

From: Indiana University website (http://kb.iu.edu/data/aibc.html)

While the "Recent" folder would be a "soft link", correct?

 

[robbieduncan]

While the "Recent" folder would be a "soft link", correct?

Not sure: I don't actually use Time Machine!

You might find the answer (or at least something of interest) in this article, especially the "Time Machine internals" section starting about 1/3 of the way down.