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My question is similar to this one except my full and differentials do not happen as often. I do a full once a week and a differential on weekday evenings. My differential backups at the beginning of the week are roughly 1/4 the size of the full backup. I know for a fact that the amount of pages/documents that are changes are much, much less than the 1/4 or so that is being backed up. Does the differential contain more data than the actual bits that have changed, or am I missing something as far as what all is changed when a file is updated? Does editing a 50 KB file result in 300 MB worth of changed bits in SQL? Those are approximations, not the actual numbers. However, the first differential being roughly 1/4 the size of the full backup is consistent, and there's also consistency in differential backups seemingly being larger than what's actually been modified. I would've tacked this on to the other question, but Stack Exchange said to avoid asking for help, and only to answer the question. I suppose this isn't necessarily a discussion board (this is my first post). For what it's worth, I've had the same results whether using Powershell or the GUI. Thanks in advance.

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2 Answers 2

Differential backups are data of all the changes since the last full backup. I think you are thinking of incremental backups, which backup since the last backup of any type. So essentially the differential backups being that size are stating that much has changed since the last time you did a full backup.

Excerpt:

If you do a differential backup more than once, it will copy all the files, or parts of files that have changed since the last full backup, even if you already have identical copies of those files in a previous differential backup.

So in conclusion, the differential backups are copying the same file for every time it's changed, which may be bigger than the original backup because it has more than one file per change.

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Mike, thanks. I understand the differentials will grow as more and more changes are made, and that they have all changes since the last full. My issue is that there is not much that changes between the full and the first differential, and yet the differential is large. I don't have definitive numbers but if that would help I would be willing to do a differential backup immediately after a full backup has finished - I'm positive the differential will be substantial in size. –  kschzhd Apr 9 '13 at 18:15
    
Did your last full backup finish successfully with no errors? Maybe not a lot was changed, but the amount at which the files were changed for each file, (5 different changes = 5 versions vs one change = 1 version) –  Mike Apr 9 '13 at 18:39
    
Yes, each of the backups finish without error, and the large size for the first differential is consistent each week. To show this, for this weekend, I have scheduled a differential backup to start immediately after the full backup completes. Both will happen on the weekend and there will be very little, if any, work done on the weekend, so there should not be file changes, and that would lead me to believe the differential would be very small in size. –  kschzhd Apr 10 '13 at 15:14
    
This weekend I set the differential backup to run immediately as soon as the full backup finished - which it did successfully. The differential size is 25.23% the size of the full backup, and it ran on Sunday (there would not have been any changes made to documents/files/sites during that time). This is what confuses me - the first differential backups seem to always be at least 25% of the full, regardless of whether there were any changes or not. Could there be some setting I have incorrectly set somewhere, either in SQL or in SharePoint? –  kschzhd Apr 15 '13 at 13:05
    
Does anyone have an idea why a differential backup run immediately after a full backup has a substantial amount of size to it? –  kschzhd Apr 16 '13 at 14:00

While taking the differential backup apart from the modifications after the full backup, it also takes some primary file references, log entries, and a number of other things.

So no much changes in data also give size of differential backup equal to full.

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