Re: Homebuilt NAS Advice
On Thu, 6 Aug 2020 11:11:41 -0400
rhkramer@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
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