Information Technology Reference
In-Depth Information
If you double-click that disk image to mount it in the Finder, you'll
see the list of folders with each stored Time Machine backup.
In any case, avoid opening files directly on your backup disk. Always
copy files to your main disk before changing them.
The Magic of Hard Links
If you look in the folder or disk image on your Time Machine backup
disk, you'll see subfolders, one corresponding to each hourly, daily, or
weekly backup. Inside each of those folders you'll find what appears
to be the entire contents of your drive. And yet, the total space
occupied on your backup disk (which you can check using the Finder's
File > Get Info command) may be only a bit larger than the space
occupied on the disk you're backing up! At first glance, this suggests
a paradox—those files should take up much more space!
Time Machine accomplishes this nifty trick using a Unix mechanism
called
hard links
. Hard links are basically pointers to a file or folder,
and those pointers take up just a tiny bit of space. (Personal Backup
also uses hard links.) You may be thinking that sounds like aliases,
but in fact they act differently. With an alias (or its Unix relative, the
symbolic link
), if you copy the alias, you get only a copy of the alias—
not of the original file; if you delete the original file, the alias no
longer functions. By contrast, a hard link behaves in almost every
respect exactly like the original file. Copy it, you get the whole file.
Delete any instance—the original file, or the hard link—and all other
instances remain.
How does this sleight of hand work? Technically, every file on your
computer is already referenced by a hard link; what's neat is that
files can have more than one hard link, so altering one doesn't affect
the others. (To learn more, consult
Wikipedia
.)
After Time Machine runs the first time (during which it copies all
your files), it simply creates a hard link to the previous version of
any folder or file that didn't change at all since the last run. That way,
it needn't keep a separate catalog of the files in your backup—the
files themselves are the catalog—and you can always get at any file
directly, should the need arise, by digging through your backup disk
in the Finder.
