One of the most frequent questions I get is to actually explain the difference between differential and incremental backups in pgBackRest.
As everyone might easily imagine, an incremental backup will only copy the database files (to keep it simple) that have been modified since the last successful backup. If you have a full backup every Sunday, and then an incremental every other day, the backup from Wednesday will be based on the one from Tuesday. That also means that if the backup from Monday or Tuesday is broken/corrupted (for any reason), you might not be able to recover from your Wednesday backup.
Now, what is a differential backup then? It is an incremental backup but it will be based on the last successful full backup. So if you have a full backup every Sunday, and then a differential every other day, the backup from Wednesday will be based on the one from Sunday! It would then doesn’t matter if the other differential backups get broken/corrupted, but since you’ll always compare what has been modified since Sunday, the size of the differential backups will be bigger over time.
Let’s have a concrete example.