In v2 there's a suggestion to use git for better efficiency than rsync,
as requested by Michał Górny:

Title: Portage rsync hardlink support
Author: Zac Medico <zmedico@gentoo.org>
Posted: 2018-07-11
Revision: 1
News-Item-Format: 2.0
Display-If-Installed: sys-apps/portage

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 [1]. 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 possible to sync more efficiently using git [2] instead
of rsync, though git consumes an increasing amount of disk space over
time because shallow pull is currently not supported [3].

[1] https://bugs.gentoo.org/660410 sys-apps/portage: use rsync
    --link-dest to implement atomic repository updates (and abort if
    signature verification fails)
[2] https://wiki.gentoo.org/wiki/Portage_Security#git-mirror_repo
[3] https://bugs.gentoo.org/552814 sync using git pull --depth=1 by
    default
-- 
Thanks,
Zac