Understanding increment chains

Definition of an increment chain

An increment chain consists of a series of incremental backups of the same path. Because you can specify any level of backup at any time, you must understand increment chains to be able to perform backups that result in the files that you want being restored.

Types of increment chains

There are two types of increment chain:

A consecutive increment chain is a sequence of incremental backups that starts with level 0 and is incremented by one. If this is how you do incremental backups, you do not need to read the rest of this section.
A nonconsecutive increment chain is one in which incremental backups skip levels or have levels that are out of sequence. An example of such a chain is one where the levels have been specified in the following order: 0, 2, 3, 1, 4, or more commonly, 0,1,1,1 or 0,1,2,1,2.

Incremental backup sources

Incremental backups base themselves on the most recent lower-level backup. For example, the sequence of backup levels 0, 2, 3, 1, 4 gives two increment chains: 0, 2, 3 and 0, 1, 4. The bases of the incremental backups are shown in the following table.

Back-up order	Incre- ment level	Incre- ment chain	Base	Files backed up
1	0	Both	Files on the filer	All files in the backup path
2	2	0, 2, 3	The level-0 backup	Files in the backup path created since the level-0 backup
3	3	0, 2, 3	The level-2 backup	Files in the backup path created since the level-2 backup
4	1	0, 1, 4	The level-0 backup, because that is the most recent level that is lower than the level-1 backup	Files in the backup path created since the level-0 backup, including files that are in the level-2 and level-3 backups
5	4	0, 1, 4	The level-1 backup, because it is both of a lower level and more recent than the level-0, level-2, or level-3 backups	Files created since the level-1 backup