On Thu, 6 Aug 2020 11:11:41 -0400 rhkra...@gmail.com wrote: > On Thursday, August 06, 2020 10:07:59 AM Celejar wrote: > > * Incremental and differential backups are backups of the delta between > > the last full backup and the current system state (either individually > > [differential] or collectively [incremental]) > > I am not Leslie Rhorer, I'm just coming out of left field, but I'd state the > above a little differently (based on the reading about backups that I've done > recently and past experiences). > > Incremental and differential backups are similar in that neither are a full > backup, they are a backup of the differences between two full images (or > backups, or mirrors). > > A differential backup (seemingly in the usage of most manufacturers) is a > backup of the differences between two images but possibly not sequential > images. (For example, if somehow a backup image is created every day, a > differential backup could be the differences between the image on say Monday > and > Thursday.) > > In contrast (and again, seemingly in the usage of most manufacturers) is a > backup of each image, for example, (in the example above), there would be an > incremental backup of the differences between the Monday and Tuesday images, > another incremental backup of the differences between the Tuesday and > Wednesday > images, and so on.
I think this is what I said, albeit expressed at greater length and more clearly ;) > > * I have no idea (nor do Google, DuckDuckGo, or Wikipedia) what a > > "decremental backup" is. > > Perhaps he means (I shouldn't put words in his mouth) a reverse backup? > Forward backups of differences are those that list the changes to make an > older > image into a newer image. You can do the reverse, and list the changes > required to make a newer image into an older image. > > Believe it or not, there are sometimes advantages to that approach, which I > won't get into very deeply. > > But one is, if you do the backups in reverse, the corrolary to that is that, > the current full image is the latest, and maybe the most likely to be needed. > > (Of course, it takes a little longer to arrange the backups in reverse.) Sure. But I'm not sure I see any significant advantage in this - slightly cheaper restores, at the cost of more expensive backups? Given that I do many, many backups, and very few restores, it hardly seems worth it. > Another is, that if you want to get rid of older backups, that is fairly > easily done by deleting the older differences. In forward backups, if you > want > to get rid of older backups, you have to create a "new" old / base image. True. But that work, of rebasing backups, you're going to be doing all the time with decremental, instead of just when pruning with normal incremental / differential, so how is this an advantage? Celejar
