Why incremental backups in Windows Vista are larger than expected

Why incremental backups in Windows Vista are larger than expected

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We’ve seen a number of customers ask why their incremental backups are almost as large as their full backup. This can be surprising on a lightly used system where you don’t change or add a lot of files between backups. Seeing files backed up that you know you didn’t change can be perplexing, but there is a good explanation: the files were indeed changed, either by a program or a perhaps inadvertently by a person. For example, any of the following actions can cause files to be changed and therefore backed up:

  • Antivirus programs writing to alternate streams on the files or programs that update media file metadata (ID3 tags, for example). Even right-clicking a file and viewing its properties can add an alternate data stream to the file, thus changing it. 
  • Changing permissions on the parent folder or setting compression.
  • Moving the files to a different folder.

Backup in Vista does not make any decisions about which types of changes to back up versus those to ignore--any file that is changed will be backed up. To determine if a file has changed, Backup looks at creation date, modification date, and last written to date (visible only programmatically). Backup does not understand any file formats and does not look in any headers to decide when to back up a file, nor does Backup use the archive bit.


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  • The Filing Cabinet has a nice explanation on this. I'm glad they wrote it since I was really beginning to wonder - why! Incremental backups are suppose to save the amount of space over the full backup - which they...

  • I actually wish that the backup program was a bit smarter. It would be nice if it would occupy a little bit less space than from 1-3 gigs each day in my case. I do not even change that many files but I guess Vista does.



  • this is not acceptable.. so what should a user do? there should be some configuration so the Backup is smarter than what you described above.. in its current form it's unusable.. so it's pointless if you have a good reason why it gets bigger than the full backup.. as a user i could care less how and why.. i just want it to work as i expect it to work based on general rules.. incremental backup is to save space.. period.. you guys should go through all the headache into creating the Backup application so it's smart enough to handle what you just explained above. the million dollar question is.. What now? the backup is growing like crazy.. and there is not even an option to delete old backups!! Right now a hobby tool, SyncToy does a much better job than the Vista Backup!!

  • I have the following 'notion' although not sure it is well founded:

    When I first began doing these automatic backups, it seemed that the incremental backups were quite large, often almost as large as the 'complete' backups.  From this article above, any little change to a file or folder will cause it to get listed for incremental backing up again, and that antivirus programs were a common cause of small changes being made.  Since I changed my antivirus program in December, 2007 from Norton to Avast, my incremental backups seem smaller and that perhaps Avast is not messing with my files as much as Norton did.  That is about as scientific as a wild guess, but I do seem to experience smaller incrementals than I used to.  Does that make sense?  Thanks.

  • Hi, John

    Yes, it is possible. Different anti-virus have different modus operandi, and it is possible that your old anti-virus was modifying more files than the new one.