The use case is simple, I want to use both SSD disks and SATA disks within the same machine and ultimately create pools pointing to SSD or SATA disks. In order to achieve our goal, we need to modify the CRUSH map. My example has 2 SATA disks and 2 SSD disks on each host and I have 3 hosts in total.

To illustrate, please refer to the following picture:

I. CRUSH Map

CRUSH is very flexible and topology aware which is extremely useful in our scenario. We are about to create two different root or entry point from which the CRUSH algorithm will go through to store our objects. We will have one root for our SSD disks and another one for our SATA disks. Looking at the CRUSH map below you will see that we duplicated our topology, it is like we let CRUSH thinking that we had two different platforms which not entirely true. We only represented a logical view of what we wish to accomplish.

Here the CRUSH map:

##
# OSD SATA DECLARATION
##
host ceph-osd2-sata {
  id -2   # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.0 weight 1.000
  item osd.3 weight 1.000
}
host ceph-osd1-sata {
  id -3   # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.2 weight 1.000
  item osd.5 weight 1.000
}
host ceph-osd0-sata {
  id -4   # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.1 weight 1.000
  item osd.4 weight 1.000
}

##
# OSD SSD DECLARATION
##

host ceph-osd2-ssd {
  id -22    # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.6 weight 1.000
  item osd.9 weight 1.000
}
host ceph-osd1-ssd {
  id -23    # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.8 weight 1.000
  item osd.11 weight 1.000
}
host ceph-osd0-ssd {
  id -24    # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item osd.7 weight 1.000
  item osd.10 weight 1.000
}

Now we create our two roots containing our OSDs:

##
# SATA ROOT DECLARATION
##

root sata {
  id -1   # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item ceph-osd2-sata weight 2.000
  item ceph-osd1-sata weight 2.000
  item ceph-osd0-sata weight 2.000
}

##
# SATA ROOT DECLARATION
##

root ssd {
  id -21    # do not change unnecessarily
  # weight 0.000
  alg straw
  hash 0  # rjenkins1
  item ceph-osd2-ssd weight 2.000
  item ceph-osd1-ssd weight 2.000
  item ceph-osd0-ssd weight 2.000
}

I create 2 new rules:

##
# SSD RULE DECLARATION
##

# rules
rule ssd {
 ruleset 0
 type replicated
 min_size 1
 max_size 10
 step take ssd
 step chooseleaf firstn 0 type host
 step emit
}

##
# SATA RULE DECLARATION
##

rule sata {
 ruleset 1
 type replicated
 min_size 1
 max_size 10
 step take sata
 step chooseleaf firstn 0 type host
 step emit
}

Compile and inject the new map:

$ crushtool -c lamap.txt -o lamap.coloc
$ sudo ceph osd setcrushmap -i lamap.coloc

Then see the result:

$ sudo ceph osd tree
# id  weight  type name up/down reweight
-21 12  root ssd
-22 2       host ceph-osd2-ssd
6 1             osd.6 up  1
9 1             osd.9 up  1
-23 2       host ceph-osd1-ssd
8 1             osd.8 up  1
11  1           osd.11  up  1
-24 2       host ceph-osd0-ssd
7 1             osd.7 up  1
10  1           osd.10  up  1
-1  12  root sata
-2  2       host ceph-osd2-sata
0 1             osd.0 up  1
3 1             osd.3 up  1
-3  2       host ceph-osd1-sata
2 1             osd.2 up  1
5 1             osd.5 up  1
-4  2       host ceph-osd0-sata
1 1             osd.1 up  1
4 1             osd.4 up  1

II. CRUSH rules

Pools configuration

Create pools:

root@ceph-mon0:~# ceph osd pool create ssd 128 128
pool 'ssd' created
root@ceph-mon0:~# ceph osd pool create sata 128 128
pool 'sata' created

Assign rules to the pools:

root@ceph-mon0:~# ceph osd pool set ssd crush_ruleset 0
set pool 8 crush_ruleset to 0
root@ceph-mon0:~# ceph osd pool set sata crush_ruleset 1
set pool 9 crush_ruleset to 1

Result from ceph osd dump:

pool 8 'ssd' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 128 pgp_num 128 last_change 116 flags hashpspool stripe_width 0
pool 9 'sata' replicated size 2 min_size 1 crush_ruleset 1 object_hash rjenkins pg_num 128 pgp_num 128 last_change 117 flags hashpspool stripe_width 0

III. OSDs configuration

Yes, you can disable updating the crushmap on start of the daemon:

[osd]
osd crush update on start = false

WoW!

IV. Bonus: HDD, SSD, Erasure code!

Thanks to David Moreau Simard, check his comment for the detailed explanations.

  # begin crush map
tunable choose_local_tries 0
tunable choose_local_fallback_tries 0
tunable choose_total_tries 50
tunable chooseleaf_descend_once 1
tunable chooseleaf_vary_r 1

  # devices
device 0 osd.0
device 1 osd.1
device 2 osd.2
device 3 osd.3
device 4 osd.4
device 5 osd.5
device 6 osd.6
device 7 osd.7
device 8 osd.8
device 9 osd.9
device 10 osd.10
device 11 osd.11
device 12 osd.12
device 13 osd.13
device 14 osd.14
device 15 osd.15
device 16 osd.16
device 17 osd.17

  # types
type 0 osd
type 1 disktype
type 2 host
type 3 chassis
type 4 rack
type 5 row
type 6 pdu
type 7 pod
type 8 room
type 9 datacenter
type 10 region
type 11 root

  # buckets
disktype osd02_ssd {
  id -4   # do not change unnecessarily
  # weight 2.000
  alg straw
  hash 0  # rjenkins1
  item osd.3 weight 1.000
  item osd.9 weight 1.000
}
disktype osd02_spinning {
  id -5   # do not change unnecessarily
  # weight 7.000
  alg straw
  hash 0  # rjenkins1
  item osd.8 weight 1.000
  item osd.17 weight 1.000
  item osd.5 weight 1.000
  item osd.11 weight 1.000
  item osd.1 weight 1.000
  item osd.13 weight 1.000
  item osd.15 weight 1.000
}
host osd02 {
  id -2   # do not change unnecessarily
  # weight 9.000
  alg straw
  hash 0  # rjenkins1
  item osd02_ssd weight 2.000
  item osd02_spinning weight 7.000
}
disktype osd01_ssd {
  id -6   # do not change unnecessarily
  # weight 2.000
  alg straw
  hash 0  # rjenkins1
  item osd.2 weight 1.000
  item osd.7 weight 1.000
}
disktype osd01_spinning {
  id -7   # do not change unnecessarily
  # weight 7.000
  alg straw
  hash 0  # rjenkins1
  item osd.0 weight 1.000
  item osd.4 weight 1.000
  item osd.12 weight 1.000
  item osd.6 weight 1.000
  item osd.14 weight 1.000
  item osd.10 weight 1.000
  item osd.16 weight 1.000
}
host osd01 {
  id -3   # do not change unnecessarily
  # weight 9.000
  alg straw
  hash 0  # rjenkins1
  item osd01_ssd weight 2.000
  item osd01_spinning weight 7.000
}
root default {
  id -1   # do not change unnecessarily
  # weight 18.000
  alg straw
  hash 0  # rjenkins1
  item osd02 weight 9.000
  item osd01 weight 9.000
}
root spinning {
  id -8   # do not change unnecessarily
  # weight 14.000
  alg straw
  hash 0  # rjenkins1
  item osd02_spinning weight 7.000
  item osd01_spinning weight 7.000
}
root ssd {
  id -9   # do not change unnecessarily
  # weight 4.000
  alg straw
  hash 0  # rjenkins1
  item osd02_ssd weight 2.000
  item osd01_ssd weight 2.000
}

  # rules
rule replicated_ruleset {
  ruleset 0
  type replicated
  min_size 1
  max_size 10
  step take spinning
  step chooseleaf firstn 0 type disktype
  step emit
}
rule volumes {
  ruleset 1
  type erasure
  min_size 3
  max_size 20
  step set_chooseleaf_tries 5
  step take spinning
  step chooseleaf indep 0 type osd
  step emit
}
rule images {
  ruleset 2
  type erasure
  min_size 3
  max_size 20
  step set_chooseleaf_tries 5
  step take spinning
  step chooseleaf indep 0 type osd
  step emit
}
rule ssd {
  ruleset 4
  type replicated
  min_size 1
  max_size 10
  step take ssd
  step chooseleaf firstn 0 type disktype
  step emit
}

# end crush map