About incremental backups

The following examples show how data is included in a series of full and incremental backups.

A differential incremental backup backs up the data that changed since the last full or differential incremental backup. Figure: Full and differential incremental example shows how data is included in a series of full and differential incremental backups between January 1 and January 4.

Figure: Full and differential incremental example

The January 1 full backup includes all files and directories in the policy backup selections list. The subsequent differential incremental backups include only the data that changed since the last full or differential incremental backup. If the disk fails sometime on January 4 (after the backup), the full backup and all three of the incremental backups are required for the recovery.

A cumulative incremental backup backs up the data that changed since the last full backup. Figure: Full and cumulative incremental example shows how data is included in a series of full and cumulative incremental backups between January 1 and January 4. The January 1 full backup includes all files and directories in the policy backup selections list. Each of the cumulative incremental backups include the data that changed since the last full backup. If the disk fails sometime on January 4 (after the backup), the full backup and the last cumulative incremental backup are required for the recovery.

Figure: Full and cumulative incremental example

Table: Retention requirements for incremental backups

Type	Retention requirement	Comments
Differential	Longer	To restore all files requires the last full backup and all the differential incremental backups that occurred since the last full backup. Therefore, all the differentials must be kept until the next full backup occurs.
Cumulative	Shorter	Each cumulative incremental backup contains all the changes that occurred since the last full backup. Therefore, a complete restore requires only the most recent cumulative incremental in addition to the full backup.

Table: Relative backup and restore times for incremental backups

Type	Backup time	Restore time	Comments
Differential	Shorter	Longer	Less data in each backup, but all differential incremental backups are required since the last full backup for a restore. This results in a longer restore time.
Cumulative	Longer	Shorter	More data in each backup, but only the last cumulative incremental is required for a complete restore (in addition to the full).

You can use a combination of cumulative and differential incremental backups together to get the advantages of both methods. For example, assume a set of schedules with the following backup frequencies and retention periods (notice that the differential incremental backups occur more often.)

Table: Example frequencies and retention periods

Backup type	Frequency	Retention period
Full	six days	two weeks
Cumulative incremental	two days	four days
Differential incremental	one day	two days

The following set of schedules result in the following series of backups:

Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7	Day 8
Full	Differential	Cumulative	Differential	Cumulative	Differential	Full	Differential

Notes about example:

Every other day a differential incremental backup occurs, which usually has a minimum backup time.
On alternate days, a cumulative incremental backup occurs, which requires more time than the differential backup, but not as much time as a full backup. The differential backup can now be expired.
To recover all files may require (at most), two incremental backups in addition to the most recent full backup. The combination of backups usually means less restore time than if all differential incremental backups were used. The full backups can be done less often if the amount of data being backed up by the incremental backups is small.