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

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

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:

More Information

Type of backup schedule attribute

How NetBackup determines that files are due for backup on UNIX

How NetBackup determines that files are due for backup on Windows

About the schedule Attributes tab