8.2. Backups

Backups have two major purposes:
  • To permit restoration of individual files
  • To permit wholesale restoration of entire file systems
The first purpose is the basis for the typical file restoration request: a user accidentally deletes a file and asks that it be restored from the latest backup. The exact circumstances may vary somewhat, but this is the most common day-to-day use for backups.
The second situation is a system administrator's worst nightmare: for whatever reason, the system administrator is staring at hardware that used to be a productive part of the data center. Now, it is little more than a lifeless chunk of steel and silicon. The thing that is missing is all the software and data you and your users have assembled over the years. Supposedly everything has been backed up. The question is: has it?
And if it has, can you restore it?

8.2.1. Different Data: Different Backup Needs

Look at the kinds of data[29] processed and stored by a typical computer system. Notice that some of the data hardly ever changes, and some of the data is constantly changing.
The pace at which data changes is crucial to the design of a backup procedure. There are two reasons for this:
  • A backup is nothing more than a snapshot of the data being backed up. It is a reflection of that data at a particular moment in time.
  • Data that changes infrequently can be backed up infrequently, while data that changes often must be backed up more frequently.
System administrators that have a good understanding of their systems, users, and applications should be able to quickly group the data on their systems into different categories. However, here are some examples to get you started:
Operating System
This data normally only changes during upgrades, the installation of bug fixes, and any site-specific modifications.


Should you even bother with operating system backups? This is a question that many system administrators have pondered over the years. On the one hand, if the installation process is relatively easy, and if the application of bugfixes and customizations are well documented and easily reproducible, reinstalling the operating system may be a viable option.
On the other hand, if there is the least doubt that a fresh installation can completely recreate the original system environment, backing up the operating system is the best choice, even if the backups are performed much less frequently than the backups for production data. Occasional operating system backups also come in handy when only a few system files must be restored (for example, due to accidental file deletion).
Application Software
This data changes whenever applications are installed, upgraded, or removed.
Application Data
This data changes as frequently as the associated applications are run. Depending on the specific application and your organization, this could mean that changes take place second-by-second or once at the end of each fiscal year.
User Data
This data changes according to the usage patterns of your user community. In most organizations, this means that changes take place all the time.
Based on these categories (and any additional ones that are specific to your organization), you should have a pretty good idea concerning the nature of the backups that are needed to protect your data.


You should keep in mind that most backup software deals with data on a directory or file system level. In other words, your system's directory structure plays a part in how backups will be performed. This is another reason why it is always a good idea to carefully consider the best directory structure for a new system and group files and directories according to their anticipated usage.

[29] We are using the term data in this section to describe anything that is processed via backup software. This includes operating system software, application software, as well as actual data. No matter what it is, as far as backup software is concerned, it is all data.