Re: Homebuilt NAS Advice
On Thu, 6 Aug 2020 10:04:28 -0500
Leslie Rhorer <lesrhorer@att.net> wrote:
> On 8/6/2020 9:07 AM, Celejar wrote:
> > On Thu, 6 Aug 2020 05:02:17 -0500
> > Leslie Rhorer <lesrhorer@att.net> 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 have no idea (nor do Google, DuckDuckGo, or Wikipedia) what a
> > "decremental backup" is.
>
> I suggest reading the DAR features list. I will post it again:
>
> http://dar.linux.free.fr/doc/Features.html
>
> Meanwhile, a decremental backup is defines as:
>
> As opposed to incremental backups, where the older one is a full backup
> and each subsequent backup contains only the changes from the previous
> backup, decremental backup let the full backup be the more recent while
> the older ones only contain changes compared to the just more recent
> one. This has the advantage of providing a single archive to use to
> restore a whole system in its latest known state, while reducing the
> overall amount of data to retain older versions of files (same amount
> required as with differential backup). It has also the advantage to not
> have to keep several set of backup as you just need to delete the oldest
> backup when you need storage space. However it has the default to
> require at each new cycle the creation of a full backup, then the
> transformation of the previous full backup into a so-called decremental
> backup. Yes, everything has a cost!
Ah, okay. So IIUC, each time you backup you do a full backup, and you then
convert the previous backups into the reverse of the more common
incremental / differential backups. I certainly don't see much
advantage to this over a typical incremental / differential backup
scheme, and why it's worth paying the cost that you acknowledge, but if
that's what you want, go for it.
> > * 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).
>
> And how useful is that? There are very few duplicate files on my
> systems, because I use applications to eliminate duplicates.
> Eliminating duplicates in a live data repository is far more important
> than doing it on backup media.
Why do you consider it far more important? And do your applications
cover the case where multiple non-identical files share some data
chunks?
> > * 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
>
> No. It is the opposite of that. It is the option to not restore files
> that have been deleted since the backup, or to actively delete files
> during a restore. It is similar - but not the same as - directing the
> restore not to restore files that do not exist on the restore target.
Okay, but I'm not really getting the use case for this. You want to
restore files, but not files that have been deleted - so that means you
only want to restore files that still exist (but have been altered)? Or
are you saying that there's some way to distinguish between accidental
and deliberate deletion?
The way I see it, if I'm restoring an entire system snapshot, I want
things back the way they were. If I want a specific file back, I know
what I want, whether I deleted it or altered it and want the original
back.
If a disk (or disk array) fails, is there any way to know what files on
it may have been deleted since the last backup?
In other words, in what real world scenario is it useful to tell the
restore program "restore everything except for files that have been
deleted since the backup"? If a disk fails, I want everything back from
the last backup, and I have no way of knowing (except for my rather
unreliable memory) what exactly has changed or been deleted since that
backup. If I have specific files that I've deleted or altered and I
want the originals back, I just restore those files manually.
Celejar
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