Comment 13 for bug 1492742

I think a percentage could work as an alternative to a cap but, in the case were you deploy your entire cluster in one shot, you will still have the problem that no one charm will have a reliable view of total amount of storage ultimately available in the cluster. Therefore, you will still need to perform some post-deployment config to optimise the number of placement groups in your pools. Nevertheless that is still a better proposition that what we have right now since you can always go up but not down (pg_num). The only benefit of a fixed rather than percentage cap is that the deployer knows how much storage they are adding so they could set the cap appropriately (although maybe "cap" is a misleading name in this case).