Now we are going to talk about the following types of backup jobs: Full, Incremental, and Differential.  Each of these terms can be used to describe a local or a remote job.

A Full backup is exactly that, entire files are backed up.  A Full backup can stand alone and represents everything you have selected for the job.

Differential jobs build off of completed full backup jobs.  Differentials compare the current state of a file to the state of the same file in the full job.  If there is a difference, the file is backed up again.  Every file that has been changed since the full is backed up.  Over time, a differential job can swell to be very close to the size of a full backup…as more and more files have been changed, they must all be backed up.  Restores are also complicated by the requirement for the original full plus the differential.  An incremental is similar, but only the differences between the current file and the previous backup are recorded. 

Incremental jobs also build off the initial full.  The files are compared, usually at the block level, to the original file in the full backup.  Any differences are recorded…but just the portions that are different, not the entire file.  The next time the incremental is run, the files are compared to the state at the last run time, and again, any differences are recorded.  So, if a job runs every 12 hours, only the changes in the last 12 hours are recorded. Restores can become complicated, because the software may need the original full plus several incrementals to reconstruct a file.  The big benefit to incrementals is that any given job can be pretty small…the full is full size, but all subsequent jobs just represent recent activity, so they are pretty small in most cases.