The Hetzner DS-8000 servers feature a 3ware RAID card (8006-2LP) for hardware RAID 1, a nice feature – although the default CentOS setup is slightly broken with SELinux. After several hours of wondering why smartd wouldn't work, /var/log/audit/audit.log reveals the following:
type=AVC msg=audit(1223922183.059:22): avc: denied { getattr } for pid=2654 comm="smartd" path="/dev/twe0" dev=tmpfs ino=8940 scontext=user_u:system_r:fsdaemon_t:s0 tcontext=user_u:
object_r:device_t:s0 tclass=chr_file
A good administrator will probably want to add some custom SELinux policies, but the lazy admin might just want to disable SELinux. I went with the disabling of SELinux as the easier option! It's not a recommended solution though.
Eventually I got smartd working again, so I've now got decent monitoring of the disks on the machine.