Differential Backup
A differential backup is one where all the changes which were made since the last full backup are stored. As opposed to an incremental backup which stores the changes made since the last incremental backup.
A simple example to illustrate. Say we have a text file with the words '1 2 3' then we do the first backup (which is a full backup as it is the first one that has been done), then the file is changed to '1 2 3 4' and differentially backed up, then it is changed to '1 2 3 4 5' then differentially backed up. The backups would be:
- '1 2 3' Full backup
- '4' Differential backup 1
- '4 5' Differential backup 2
A differential backup is seen as a half way house between a full backup and an incremental backup, so generally restoring a differential backup is quicker than an incremental backup but slower than a full backup. Also a differential backup will use more space than an incremental backup but less than a full backup. When considering which option to go for it tends to be the case that if the larger changes between the file size between backup is the more tending towards differential and full backup, and the smaller that the changes are the tending is towards differential and incremental backup.
Advantages of a differential backup
- Less storage is needed than when doing a full backup
- Backing up takes less time than when doing a full backup
- Restoration is faster than using an incremental backup strategy
Disadvantages of a differential backup
- Backing up takes more time than with an incremental backup
- Restoring files takes longer than with a full backup
- More storage space is required with this strategy than with incremental backup
Improvements which some software vendors do to mitigate the disadvantages
- To lessen the effect of a long backup time, some vendors provide the ability to trigger a full backup in the case where the differential backup file size would exceed a certain size