Is there any way to restore/create hardlinks lost in incremental backups?

[Chris Green]

Paul Slootman via rsync <rsync at lists.samba.org> wrote:
> On Thu 10 Dec 2020, Chris Green via rsync wrote:
> > 
> > Occasionally, because I've moved things around or because I've done
> > something else that breaks things, the hard links aren't created as
> > they should be and I get a very space consuming backup increment.
> > 
> > Is there any easy way that one can restore hard links in the *middle*
> > of a series?  For example say I have:-
> > 
> >     day1/pictures
> >     day2/pictures
> >     day3/pictures
> >     day4/pictures
> >     day5/pictures
> > 
> > and I notice that day4/pictures is using as much space as
> > day1/pictures but all the others are relatively small, i.e.
> > day2 day3 and day5 have correctly hard linked to the previous day but
> > day4 hasn't.
> > 
> > It needs a tool that can scan day4, check a file is identical with the
> > one in day3 then hardlink it without losing the link from day5.
> 
> If you have these files that are hardlinked:
> 
>     day1/pictures/1.jpg
>     day2/pictures/1.jpg
>     day3/pictures/1.jpg
> 
> And these are hardlinked, but to a different inode:
> 
>     day4/pictures/1.jpg
>     day5/pictures/1.jpg
> 
> then there is no way of linking the second group to the first in one
> step; you will have to individually link day3/pictures/1.jpg to
> day4/pictures/1.jpg and then day3/pictures/1.jpg (or
> day4/pictures/1.jpg) to day5/pictures/1.jpg.
> 
> It's not like a group of directory entries that are hardlinked to one
> inode are some sort of actual group; they just happen to be directory
> entries that point to the same inode number. There is no other relation
> between those directory entries.
> 
> So you will have to incrementally process each next day against the
> previous day.
> 
Yes, that's what I have done, wrote a trivial[ish] script that copied
all the backups to a new destination sequentially (using --link-dest)
and then removed the original tree, having checked the new backups
were OK of course.

Fortunately I have lots of spare space on the backup system at the
moment having just upgraded it with a new 8Tb drive, so duplicating
the whole backup wasn't an issue (though rather slow because it was
from and to the same drive).

> 
> If I make a significant change in such a directory structure (e.g.
> renaming a directory) I try to remember to do the same thing on the
> backup which some say is wrong, but it saves a lot of space, like you
> discovered :)
> 
Yes, I've sometimes done that.

-- 
Chris Green
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