Time Machine file system

[vertisync]

I have a question about the way Time Machine puts files on an external hard drive. I have been using it for a couple months and I am just curious how it work out of interest (nothing is wrong).

When I look through my external drive in which Time Machine backs up to, I see all the files under the various times and dates that Time Machine saved them to. But I thought it only did incremental backups? As in, it only backs up things that have changed? It seems as if it's got all the full files in each different backup dated folder.

So I am just curious how it works.

 

[chown33]

It uses hard links:
http://en.wikipedia.org/wiki/Hard_link

So if a file is unchanged, the new backup is just a hard link to the previous backup's file. Two filenames in two different directories, but they refer to the same actual data on disk (i.e. a hard link).

A hard link differs from a symlink:
http://en.wikipedia.org/wiki/Symlink

 

[vertisync]

That makes perfect sense.

So if I wanted to manually go into the drive and copy a file (without going into the Time Machine app) I could copy the file from any directory and it will work, even if it is a hard link?

 

[chown33]

That makes perfect sense.

So if I wanted to manually go into the drive and copy a file (without going into the Time Machine app) I could copy the file from any directory and it will work, even if it is a hard link?

The copy would work. A hard link is a real file (or directory). It's not a symlink.


If you did go in and copy a file, you might get more than you expected or wanted. I vaguely recall that Time Machine puts either some ACLs or some extended attributes on its backups, that prevent people from going in and mucking about. If you were to copy a file without going thru Time Machine, you might encounter that, and it could lead to difficulties of one kind or another. That's my vague recollection, though, and I could be wrong.

Basically, if you want to copy things from Time Machine backups, your best bet is to use Time Machine itself to copy things out.

 

[vertisync]

That's good to know, I will do that. Thanks a lot for your help and info.

 

[Bomb Bloke]

You're indeed better off using the Time Machine interface, but copying files out manually does indeed work. Handy if the original system dies and you want to get just a few specific files using another computer (in which case the regular Time Machine interface isn't an option).

Do not attempt to remove anything from the backup's file structure without using the Time Machine interface to do so. This has the potential to not only corrupt the backup, but the Trashes folder of the drive that holds it.