We help IT Professionals succeed at work.

Differential Backups

LFC1980 asked

If you have a normal backup carried out every friday, then a differential backup carried out on monday-thrusday.

Then supposing you had some data loss on a thursday so needed to restore to wednesdays data.

Would start with the normal tape from the prev friday and work your way up from Mondays tape, then tuesdays, and finally wednesday?.

Or do you only need the normal tape from the friday and just the single tape from wednesday?
Watch Question

Top Expert 2007
You start with the full from Friday and then restore the diff from Wednesday.
Distinguished Expert 2019

Depeding on the application and the options available to you.  i.e. you select the point in time for a particular file/files/directories you want restored.  once you select those items, the software will tell you which tapes it needs.  If you know  the files in questions were altered on Wednesday, you can use Wednesday differential, locate the files and copy them back if that option is available to you in the backup application.  

If you are unsure, depending on how the differential backup data is provided might do the reverse from sibounty's recommendation.  Go from the most recent differential up until the full.  Make note of the files you restore so that the older versions will not ovewrite the recently restored.
Lee W, MVPTechnology and Business Process Advisor
Most Valuable Expert 2013

sirbounty is correct.  A differential backs up all data that has changed since the last full.  So assuming the data selection is the same (for example, you're backing up the entire D: drive on your server with both the full and differential) , then you restore the full and the latest differential only.  You would NOT restore the other differentials UNLESS the latest is corrupt OR files have been deleted that you want to recover.

What arnold seems to be describing is not a differential, but what is called an incremental.  Incrementals backup ONLY data that has changed since the last backup - FULL OR PREVIOUS INCREMENTAL.  So using INCREMENTAL backups, your backup sets would be smaller (than the full), but you would need to restore EVERY individual Incremental to completely restore the server.  

You might want to read over my backup comment - which got so long and in depth, I turned it into a web page.  I cover backup types - among other things - in the article.
Top Expert 2007

Had to run away for a moment.

Basically the way it works is - a full backup clears an attribute (bit) on each file as it's backed up.
As those files are modified, that bit is reset.
This is how the differential determines whether the file needs to be backed up or not.  It backs up those files that have the bit turned off, but it doesn't turn it back on.
An incremental backup 'will' turn the bit back on, which is why it takes potentially more tapes to restore from an incremental backup...


Cheers guys

Knew it was that, but got a different answer from these brainbump questions, so thought i'd ask