Author: MatthewHyclak
Let's start with an example directory:
[user@machine /tmp/foo]$ ls -l total 108 -rw-r--r-- 1 user group 67584 Apr 6 08:47 a -rw-r--r-- 1 user group 6144 Apr 6 08:47 b -rw-r--r-- 1 user group 28672 Apr 6 08:47 c
Before we can do anything else, we must do a Level 0 backup of our /tmp/foo directory, so the files a, b, and c are all included in our Level 0.
[user@machine /tmp/foo]$ ls -l total 108 -rw-r--r-- 1 user group 67584 Apr 6 08:47 a -rw-r--r-- 1 user group 6656 Apr 6 08:51 b -rw-r--r-- 1 user group 28672 Apr 6 08:47 c
The next time we do a backup, we don't necessarily need every file if they haven't changed. So we can do an incremental backup, in this case a Level 1. Looking at the listing above, we can see that only file b has changed. That means that our Level 1 backup only contains file b. If we want to restore the entire /tmp/foo directory, we'll have to use both our Level 0 and our Level 1 backups.
[user@machine /tmp/foo]$ ls -l total 140 -rw-r--r-- 1 user group 67584 Apr 6 08:47 a -rw-r--r-- 1 user group 6656 Apr 6 08:51 b -rw-r--r-- 1 user group 56832 Apr 6 08:53 c
The next time we do a backup, we have a choice.
We can do another Level 1, in which case everything that has changed since the last Level 0 will be saved. In this case that would include files b and c.
We can do a Level 2, in which case everything that has changed since the last Level 1 will be saved. In this case that would only include file c.
What difference does it make? Well, there are tradeoffs. If we do a Level 2, our backup size will be smaller since we're only including one file. In our example here, that doesn't make much difference, but if you are backing up a 4 TB system, it could be a big deal. The flip side is we need more tapes to restore the data. To restore data completely, you need the Level 0, and the most recent of each of the additional levels you have done. In this case we would either need a Level 0, Level 1, and Level 2, or if we opted to do two Level 1s, we could get away with two tapes; Level 0 and the latest Level 1.