Question about NAS differential/incremental backups

February 28, 2013 at 13:00:30
Specs: Windows Server 2003SP2, 2.99 ghz, 4GB Ram
I understand that a full backup backs up everything, and a differential backs up all changes since a full backup, and an incremental backup backs up all changes since the previous incremental.

My belief is that I can do a full backup on Sunday, then differentials Mon- Fri. Then, if everything crashes, I can restore the full, then the most recent differential (As opposed to a full restore, an a bunch of incrementals to catch up)

My question is if I do a full backup on Sunday, on Tuesday user Mary creates a file, on Wednesday she deletes it by mistake, then on Friday she realizes it and calls me.

Will that file still exist on Thursday night's differential? Or do I need to load up Wednesday night's backup?

The reason I am asking is because I am trying to go off of tapes and use a NAS instead. I would like to have a Sunday full backup, then have one differential backup that gets overridden nightly, to save space.

By doing this, I will only have a full backup and the most recent differential.

The alternative is to have a weekly backup and 5 differentials, which takes up more space, but I am more protected against morons.


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February 28, 2013 at 13:31:39
A differential backup doesn't back up all changes since a full backup. It backs up all files now on the system that have changed or are new since the last full backup. That is not the same thing. On Thursday, when the differential backup is taken, Mary's file no longer exists; it didn't exist on Sunday. So, no change; it doesn't get backed up. (Difficult to see how it could since it doesn't exist anywhere on the system.) You need to restore the file from Wednesday's differential (which can mean a bit of searching if you are not sure when it was deleted).

The idea of full and differential/incremental backups is out-of-date, IMO. The better backup systems continually backup files and will present you with the latest version of files plus a number of previous versions. In the home environment an example of this is Time Machine on OS X or Genie Timeline on Windows. When I worked as a Network Manager I ran an enterprise version of this breed of software called Tivolli Storage Manager. It backed up to disks on a server which were then backed up to tapes in an automated tape library. Copies of those tapes were then stored offsite.

Recent versions of files would almost always be still on disk and so could be restored within about 5 minutes of receiving the request. This contrasted with the day or so that it took with our previous system to get the tapes back from an offsite location and then load them up and search them for the file. The central server backed up about 30 other servers.

The users thought it was magic, and that I was a hero! They were very nearly right.

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