ACRONIS Backup & Recovery 10 Advanced Server User Manual page 33

Example: An Internet cafe, school or university lab where the administrator often undoes changes
made by the students or guests but rarely updates the reference backup (in fact, after installing
software updates only). The backup time is not crucial in this case and the recovery time will be
minimal when recovering the systems from the full backup. The administrator can have several
copies of the full backup for additional reliability.
Incremental backup
An incremental backup stores changes to the data against the latest backup. You need access to
other backups from the same archive to recover data from an incremental backup.
An incremental backup is most useful when:
 you need the possibility to roll back to any one of multiple saved states
 the data changes tend to be small as compared to the total data size.
It is widely accepted that incremental backups are less reliable than full ones because if one backup
in the "chain" is corrupted, the next ones can no longer be used. However, storing multiple full
backups is not an option when you need multiple prior versions of your data, because reliability of an
oversized archive is even more questionable.
Example: Backing up a database transaction log.
Differential backup
A differential backup stores changes to the data against the latest full backup. You need access to
the corresponding full backup to recover the data from a differential backup. A differential backup is
most useful when:
 you are interested in saving only the most recent data state
 the data changes tend to be small as compared to the total data size.
The typical conclusion is: "differential backups take longer to do and are faster to restore, while
incremental ones are quicker to do and take longer to restore." In fact, there is no physical difference
between an incremental backup appended to a full backup and a differential backup appended to
the same full backup at the same point of time. The above mentioned difference implies creating a
differential backup after (or instead of) creating multiple incremental backups.
An incremental or differential backup created after disk defragmentation might be considerably larger than
usual because defragmentation changes file locations on the disk and the backup reflects these changes. It is
recommended that you re-create a full backup after disk defragmentation.
The following table summarizes the advantages and shortcomings of each backup type as they
appear based on common knowledge. In real life, these parameters depend on numerous factors
such as the amount, speed and pattern of data changes; the nature of the data, the physical
specifications of the devices, the backup/recovery options you set, to name a few. Practice is the
best guide to selecting the optimal backup scheme.
Parameter
Storage space
Creation time
Recovery time
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Full backup
Maximal
Maximal
Minimal
Differential backup
Medium
Medium
Medium
Incremental backup
Minimal
Minimal
Maximal
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