A differential backup is a type of backup that copies all the data that has changed since the last full backup. For example, if a full backup is done on Sunday, Monday's differential backup backs up all the files changed or added since Sunday's full backup. The differential backup performed on Tuesday then backs up all the changed files since Monday's full backup, including the files changed on Tuesday -- and so on for the rest of the week.
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If a full restore is needed at some point in time, only the last full backup and latest differential backup are required.
Pros and cons of differential backups
Just like any other technology, there are pros and cons to differential backups. The process is faster than a full backup and only backs up the data with changes, so it saves storage space.
However, the data changes that are backed up in a differential backup become progressively larger with each full backup cycle. If the cycle is too long, the size of the archive will be larger and make the process longer.
Other cons are that a full backup is required before the differential backup process can begin. And if either the full or differential backup fails, the data recovery process cannot be completed.
Differential backup vs. incremental backup
An incremental backup differs from a differential backup in that it only takes a copy of the data changed or added since the last backup -- whether that was a full backup or an incremental backup.
With an incremental backup, fewer files are copied than with a differential backup. However, the restore process may take longer since the software needs to rebuild the data from the most recent full backup as well as the separate incremental backups.
See also: cloud backup