The autoscaler [1] will be supported during RHCSv4 (though not on by default). This is a request for ceph-ansible to support deployments with this feature this as PG management causes field escalations.

One way this could work is if there's a data structure passed like this:

openstack_pools:
    - {"name": backups, "target_size_ratio": 0.1, "pg_autoscale_mode": True, "application": rbd}
    - {"name": volumes, "target_size_ratio": 0.5, "pg_autoscale_mode": True, "application": rbd}
    - {"name": vms,     "target_size_ratio": 0.2, "pg_autoscale_mode": True, "application": rbd}
    - {"name": images,  "target_size_ratio": 0.2, "pg_autoscale_mode": True, "application": rbd}

Then ceph-ansible could execute commands like this for each pool (e.g. this is for the volumes pool). 

    $ ceph osd pool create volumes 16
    $ rbd pool init volumes
    $ ceph osd pool set volumes target_size_ratio .5
    $ ceph osd pool set volumes pg_autoscale_mode on

The target_size_ratio is like the percentages customers input into pgcalc. Putting it directly into THT and skipping pgcalc is a better UX.

The 16 in the first command was recommended as a hard coded value above while the name and target_size_ratio would be variables. The reason for 16 was based on Sage's blog [1] where he set 1, but 16 is the usual default and should work with replicated pools while not triggering the overdose protection.

[1] https://ceph.io/rados/new-in-nautilus-pg-merging-and-autotuning/