Portage rsync hardlink support
For users of the rsync tree, beginning with sys-apps/portage-2.3.42, the default behavior for sync operations will use hardlinks in order to ensure that a repository remains in a valid state if something goes wrong . For example, if signature verification fails during a sync operation, the new hardlink behavior will preserve the previous state of the repository.
The new behavior may conflict with configurations that restrict the use of hardlinks, such as overlay filesystems. Therefore, users will have to set "sync-allow-hardlinks = no" in repos.conf if they have a configuration that restricts the use of hardlinks, but this should not be very common:
[DEFAULT] sync-allow-hardlinks = no
Note that it is possible to sync more efficiently using git  instead of rsync, though git consumes an increasing amount of disk space over time unless shallow pull is enabled via the sync-depth option in repos.conf  (requires sys-apps/portage-2.3.42 or later).
--link-dest to implement atomic repository updates (and abort if signature verification fails) git pull by setting sync-depth = 1 in repos.conf