Why are differential backups larger than full?

I have a full backup and I have made two differntial backups since then. The full backup is approximately 17.5 GB. The first differential backup is about 22 GB and the next differential backup is 24 GB. Why would this be? Shouldn't differential backups be much smaller that a full backup?

 

In theory, each successive DIFF will be larger than the previous one.
Will they be larger than the FULL? I suppose if you defragged the drive and added a number of files, they could be.

 

I understand that each successive differential file will be larger than the previous one, but I don't understand why they are larger than the original. I have not defragged the drive and added some programs and files, but shouldn't a differential backup only backup those changes? How in the world would that be bigger than the original full backup?

 

A differential backup contains all files that have changed since the last FULL backup. The advantage of a differential backup is that it shortens restore time compared to a full backup or an incremental backup. However, if you perform the differential backup too many times, the size of the differential backup might grow to be larger than the baseline full backup.

Personally, I do only FULL backups. I don't like backups that depend on other backups.

 

“A differential backup contains all files that have changed ...”

Not to get picky, but ...

In the case of TI, this is not exactly the case. TI will backup any sector changes. Even if no files have been altered, if they have been moved, TI will include those changes in a DIFF image.

 

Thanks for responding, but I realize that. I know what a differential backup is. Again, I have three backups.

#1) FULL Backup 17.5GB - made on 9/23
#2) DIFF Backup 22 GB - made on 10/28
#3) DIFF Backup 24 GB - made on 10/30

NOTE here. The SECOND backup and the FIRST differential backup is larger than the FULL backup. I added some programs and therefore some files. But the DIFF backup should only have backed up THOSE changes. In NO way would those changes be bigger than the FULL backup.

 

I would rather do differential backups than incremental, but if this is what is happening, I might as well do incremental.

 

Are you sure you are creating DIFF images?
If you select a target for the DIFF and the FULL is not in that folder (or has been renamed), TI will create a FULL instead of a DIFF (same with INCs).

 

I fully agree with you. It doesn't make sense that the second backup is greater than the first backup in size. I can't explain it either. There is probably a technical explanation for this, but as a simple user I don't care because it is UNLOGICAL that this happens.
I hope for you that incremental backups will take much less space than the first backup, otherwise there is something really wrong with ATI.

 

I am positive that I am creating DIFF images. The FULL IS in the same target folder. This is why I don't understand what is going on here.

 

What are the file names of all 3 images?

 

I think TheWeaz is looking in the right direction.

Also: if in Validate you click on each of the three files in turn, will it be reported (top right) as "This is Volume 1 (resp. 2, 3) of a multivolume image archive" ?

Or will it just say: "This is image archive" ?

 

OK, the names of the three are:

FULL: TrueImage 92306.tib - When I made this, it was on 9/23/06. So I named it "92306"

1st DIFF: TrueImage 923062.tib - This name was automatically generated and the "2" added at the end.

2nd DIFF: TrueImage 9230623.tib - Again, this name was automatically generated and the "3" added at the end of the original name.

 

It seems that TI did create proper differentials appended to the full image as you meant it to. My guess that due to some mistaken operator selection two more full images were created, is therefore not confirmed.

Unless you have defragmenting running in the background (on idle) I have no idea what would make TI include in the first diff all the sectors already imaged in the full plus the programs you installed after creating the full.

I would make another test, with differentials as well as incrementals. To diagnose, I would create the first differential/incremental soon after the full and the second one after the computer has been idle for a couple of hours. If there is a reason for a diff to turn out large, it should apply to the parallel incremental as well.

You may also want to check Windows System Restore settings.

 

THe differential has to include structures that keep track of the changes, so it can be bigger than a full.

 

I didn't know this.
I knew there was a reason why I always preferred making strictly FULL Backups. Usually do one every week.

 

That could happen, yes, but according to the info supplied by hoser_d this is not a case where it should.

 

I would say that was only likely if the layout of the in-use sectors had changed hugely in between a Full and a Differential (e.g. a disk defrag had taken place).

Regards