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According to this article from Microsoft

  • Differential backups require even less disk, tape, or network drive space than incremental backups.
  • Backup time is faster than full or incremental backups.

How is this true because isn't there more files to backup since it's doing everything from last full backup where incremental only does files changed since last incremental backup?

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up vote 1 down vote accepted

Differential backup backups only the changes from the last backup , storing only the difference. This scheme makes it use much less space on an average from an incremental backup which stores the complete file which was modified.
Checkout the wikipedia article for more on the same

-- Update--
The reason that lower space is used is because only the differences between two versions are stored instead of a snapshot of the whole database. The logic behind this is that over different versions actual changes are very small as compared to the size of the actual file.
Imagine a manuscript that an author is editing, the actual changes everyday might be just a few hundred kbs whereas tha actual size of the file might go into a couple of mbs.Ths incremental backup would contain the 2 mb snapshot whereas a differential backup would only be 100-200 kb. This would allow lesser space to be consumed as well as faster backup, where only tha change would be inserted into the actual file

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I thought the same at first, but the linked Microsoft KB article describes it in a way that matches James's answer: "A differential backup provides a backup of files that have changed since a full backup was performed.". I gather 'differential' is one of those words for which Microsoft has its own definition, and cares nothing for anyone else's. – Aaron Miller May 6 '13 at 20:33
@AaronMiller that's what I'm wondering, is Microsoft's definition nonstandard? – Celeritas May 6 '13 at 21:25
@Celeritas Yup. – Aaron Miller May 6 '13 at 21:32

Think what they are saying is you would only ever have one differential backup

  • Differential = since last full backup
  • Incremental = since last incremental

Therefore if a file changes twice then it could be backed up twice by incremental, but only once by differential. Differential will only save essentially the latest version of file.

EDIT: For wikipedia reference

Microsoft, makers of Backup and Restore, defines incremental backups and differential backups as follows: "Incremental: An incremental backup provides a backup of files that have changed or are new since the last incremental backup....Differential: A differential backup provides a backup of files that have changed since a full backup was performed. A differential backup typically saves only the files that are different or new since the last full backup, but this can vary in different backup programs. Together, a full backup and a differential backup include all the files on your computer, changed and unchanged."

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