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

[Paul Slootman]

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.


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 :)


Paul