Hi everyone. Now that we're on the subject of hard linked files, here's my regular reminder to all the mirror admins to use the -H flag when rsyncing from msync.centos.org. The -H flag preserves hard links, which are already extensively used for CentOS content. One way to verify that your hard links are OK is to run "stat 6.10/os/x86_64/images/boot.iso". There should be "Links: 2" (or more) in the output, because boot.iso is the same file as 6.10/isos/x86_64/CentOS-6.10-x86_64-netinstall.iso. Using hard links makes syncs faster and saves hard disk space.
If you just added -H to your rsync command line, rsync will take care of deleting the unneeded copies of hard linked files automatically the next time you sync.
And now on to João's specific concerns.
We do run "hardlink" regularly on the master server, but we do so without the -c flag which "Disregards permission, ownership and other differences" [such as modification time].
The repodata files need to preserve their modification times, because the timestamp is included in repomd.xml. If hardlink changes the modification time of a file mentioned in repomd.xml, it may cause odd problems.
The drpms are easier in this regard and yes, it might make sense to run hardlink on those because the exact timestamp is less important for drpms (as far as I'm aware). I don't have the authority to do so, however, so it would need to be someone else.
But the drpm issue may soon be a moot point. There are plans to drop drpms altogether [1] and without drpms, there won't be a need to hard link them either.
[1] https://lists.centos.org/pipermail/centos-devel/2019-June/017433.html