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Sequential differential backups sometimes smaller in size.

I recently installed Aomei Backupper Pro 4.1.0 and am using the space disk management scheme. Sometimes the file for a differential backup is smaller then the previous differential backup. How is this possible?


  • Can you upload a screenshot?
  • Differential backup 13 is smaller than 12. Is this possible or an error of some sort?

  • Differential backups keep track of all information that changes since the last full backup.  There are different modes of differential backups.  If a differential backup is set to "append", the backup will contain files that have been added and/or changed but files deleted on the source will remain in the backup data set, resulting in sequential backups that are consecutively larger.

    In the case of AOMEI, it appears (in my experience) that differential backups use what is commonly referred to as "overwrite" or "mirror" mode, meaning that the backup data set not only includes files that have been added and/or changed but also excludes files that have been deleted on the source.  Deleting a large amount of data on the source in between differential backups could absolutely result in smaller backup sizes.
  • Thanks. That had occurred to me, although I don't recall deleting 22GB. Possibly something in the recycle bin that finally got flushed. The smaller file verified OK, so I guess I'll assume all is well......
  • I don't recall deleting 22GB

    Maybe windows.old, the previous version of Windows10 kept for a few weeks after a big Win10 update. After a few weeks,and you did not rollback, the windows.old is deleted automatically.
  • @JohnnyboyGo

    That's exactly where I was going to go.  Windows 10 version 1803 update rolled out in April.  It would be about due for being auto-purged now.
  • That's a great insight. Thanks!
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