The default rules are sane for small clusters with few failure domains.
Anything larger than a single rack should customize their rules.
It's a good idea to figure this out early. Changes to your CRUSH rules can
result in a large percentage of data moving around, which will make your
cluster unu
Ceph uses CRUSH (http://ceph.com/docs/master/rados/operations/crush-map/) to
determine object placement. The default generated crush maps are sane, in that
they will put replicas in placement groups into separate failure domains. You
do not need to worry about this simple failure case, but you
