Backup Systems & Scheduling - Normal, Differential, Incremental, Oh My!

In the last article of this series we examined the reasons why data backups are so important, along with different methods for storing backups, and some of the tools and utilities involved. In this article we are going to go into the different types of backups out there and how to properly utilize them in order to have the most reliable storage, up to date backups, and an organized system that is easy to maintain.

How often you perform backups is going to depend entirely on how often the data on your server is modified. If parts of it are modified on a daily basis, then you are going to want daily backups. If it's only every couple of days, then the number of backups will be less.

The size of the storage medium that you are going to need is going to also depend on the type of data and how much of it is needing to be backed up regularly. It is likely that you will use different storage media for different types of backups.

Unless you've done a lot of backups in your time you've probably never heard of an Archive Bit. An archive bit is, essentially, a tag that is attached to every file. In actuality, it is a binary digit that is set on or off in the file, but that's crummy technical jargon that doesn't really tell us anything. For the sake of our discussion, just think of it as the flag on a mail box. If the flag is up, it means the file has been changed. If it's down, then the file is unchanged.

Archive bits let the backup software know what needs to be backed up. The differential and incremental backup types rely on the archive bit to direct them.

Normal

The "normal" backup type is the most standard. This is the backup type that you would use if you wanted to backup every file in a given folder or drive. It backs up everything you direct it to regardless of what the archive bit says. It also resets all archive bits (puts the flags down). Most backup software, including the built-in Windows backup software, lets you select down to the individual file that you want backed up. You can also choose to backup things like the "system state".

Incremental

When you schedule an incremental backup, you are in essence instructing the software to only backup files that have been changed, or files that have their flag up. After the incremental backup of that file has occurred, that flag will go back down. If you perform a normal backup on Monday, then an incremental backup on Wednesday, the only files that will be backed up are those that have changed since Monday. If on Thursday someone deletes a file by accident, in order to get it back you will have to restore the full backup from Monday, followed by the Incremental backup from Wednesday.

Differential

Differential backups are similar to incremental backups in that they only backup files with their archive bit, or flag, up. However, when a differential backup occurs it does not reset those archive bits which means, if the following day, another differential backup occurs, it will back up that file again regardless of whether that file has been changed or not.