Ceph » RADOS

http://ceph.com/docs/master/rados/operations/crush-map/

rule platter {
ruleset 3
type replicated
min_size 0
max_size 10
step take platter
step chooseleaf 0 type host
step emit
}

rule ssd {
ruleset 4
type replicated
min_size 0
max_size 10
step take ssd
step chooseleaf 0 type host
step emit
}

rule ssd-primary {
ruleset 4
type replicated
min_size 0
max_size 10
step take ssd
step chooseleaf 1 type host
step emit
step take platter
step chooseleaf -1 type host
step emit
}

Using this example it could happen that the primary and one replica end up on the same host if a host contains both ssd and hdd drives. It should be possible to write crush rules which ensure that the data will always be spread over n-replicas hosts, even when a host has sdd and hdd drives.