22 Copyright © Acronis, Inc., 2000-2009It is widely accepted that a full backup is the slowest to do but the fastest to restore. With Acronistechnologies, recovery from an incremental backup may be not slower than recovery from a full one.A full backup is most useful when:• you need to roll back the system to its initial state• this initial state does not change often, so there is no need for regular backup.Example: An Internet cafe, school or university lab where the administrator often undoes changesmade by the students or guests but rarely updates the reference backup (in fact, after installingsoftware updates only). The backup time is not crucial in this case and the recovery time will beminimal when recovering the systems from the full backup. The administrator can have several copiesof the full backup for additional reliability.Incremental backupAn incremental backup stores changes to the data against the latest backup. You need access toother 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 inthe "chain" is corrupted, the next ones can no longer be used. However, storing multiple full backupsis not an option when you need multiple prior versions of your data, because reliability of anoversized archive is even more questionable.Example: Backing up a database transaction log.Differential backupA differential backup stores changes to the data against the latest full backup. You need access to thecorresponding full backup to recover the data from a differential backup. A differential backup ismost 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, whileincremental ones are quicker to do and take longer to restore." In fact, there is no physical differencebetween an incremental backup appended to a full backup and a differential backup appended to thesame full backup at the same point of time. The above mentioned difference implies creating adifferential backup after (or instead of) creating multiple incremental backups.An incremental or differential backup created after disk defragmentation might be considerably larger thanusual because defragmentation changes file locations on the disk and the backup reflects these changes. It isrecommended that you re-create a full backup after disk defragmentation.The following table summarizes the advantages and shortcomings of each backup type as theyappear based on common knowledge. In real life, these parameters depend on numerous factorssuch as the amount, speed and pattern of data changes; the nature of the data, the physical
