Some trees contains a symlink for their iso/ directory, linking to an absolute path under /mnt. This is a broken symlink under normal circumstances but it is served correctly by the HTTP/FTP mirror servers because they have the necessary mounts which the symlink is assuming.
$ ls -al /net/example.com/vol/engineering/devarchive/redhat/rel-eng/RHEL6.5-20131213.0/6/Server/ppc64/
drwxr-xr-x. 3 root root 172032 Dec 13 11:06 debug
lrwxrwxrwx. 1 root root 58 Dec 13 11:46 iso -> /mnt/redhat/iso/work/RHEL6.5-20131213.0/6/Server/ppc64/iso
drwxrwsr-x. 2 root root 4096 Dec 13 11:46 jigdo
drwxr-xr-x. 8 root root 4096 Dec 13 11:18 os
In this case, beaker-import imports an nfs+iso:// URL successfully, but it is not usable by Anaconda since it will see the link as broken over NFS, and it causes beaker-expire-distros to complain constantly about the URL being missing.
The failure mode would actually be worse if NFS *did* allow symlinks spanning to other mounted volumes: we could end up trying to do a provision over the WAN.
Time for a new architecture guide section on distro import, storage and URLs, perhaps? Bug 978824 touches on this.
I think we need to do something about this soon (even if it's just a hack in the existing import script, rather than a nicer solution using the recursive importer we had planned). It causes a huge amount of cron spam and we regularly import distros that are not installable due to the symlink.