Information Technology Reference
Why Create Versioned Backups?
Time Machine and most other backup programs protect data by using
versioned backups—that is, backing up your files without overwriting
or deleting earlier versions already stored on your backup media.
The first time your backup software runs, it copies all your files in
their entirety; and then on subsequent runs it performs an incremental
update—that is, it copies only new or changed data. In some cases,
incrementally updating a backup means copying each file that has
changed in its entirety; in others, backup applications copy only the
changed portions of files. The latter approach, which I refer to as Delta
Encoding , is faster and uses less storage space.
Note: In an earlier incarnation of this topic I coined the term “sub-
file updating” to refer to incremental updates that copy only the
changed blocks or bytes of a file, but I've now switched to saying
“delta encoding,” which is a more common term.
You might be tempted to believe that all those extra versions of your
files are a waste of space, but because both humans and computers
make mistakes, this type of backup can come in extremely handy.
Let's say, for the sake of discussion, that your only backup is a
duplicate of your entire disk that you update each Wednesday. On
Tuesday, you accidentally delete a file, but you don't realize until
Thursday. Too bad: it's not in your backup, because in the process
of duplicating your disk, you deleted any files on the duplicate that
weren't on the original. Ironically, the more frequently you update
your duplicate, the greater the chances of encountering this problem!
Or consider another situation. A buggy application writes some data
to the wrong place, damaging numerous files. Again, you don't realize
right away that there's a problem, and you update your duplicate. Sure,
you have a backup, but it's a backup of a corrupted disk!
You may not notice a missing or damaged file for weeks or months. So
it pays to maintain versioned backups that go back as long as possible
(for practical reasons, a year's worth is probably enough).