Re: Homebuilt NAS Advice
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 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.)
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.
There is more.
>
> * Borg doesn't fit the "full / differential / incremental" paradigm
> neatly, but it certainly has some of the advantages of differential /
> incremental backups (plus others that classic differential /
> incremental ones do not have, such as deduplication - i.e., if the
> backup source contains multiple copies of some data, that dataonly
> needs to be stored once in the backup target).
>
> * I don't know exactly what you mean by "deletion restoral", but Borg
> (and, I assume, many other good backup solutions) offers a
> flexible variety of methods to restore deleted files, and / or to
> restore a snapshot of the backup source as it existed at a given point
> in time: https://docs.borgbase.com/restore/
>
> If you are willing to explain further, that would be appreciated.
> Thanks!
>
> Celejar
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