Description of problem:
The 1.3 Ceph Storage strategies documentation states:
https://access.redhat.com/documentation/en/red-hat-ceph-storage/version-1.3/red-hat-ceph-storage-13-storage-strategies/#erasure-code
_____________________________________________________________________________
Chapter 30. Erasure-coded Pools and Cache Tiering

Erasure coded pools require more resources than replicated pools and lack some functionality such as partial writes. To overcome these limitations, we recommend to set a cache tier before setting the erasure-coded pool.

For instance, if the pool hot-storage is made of fast storage:

$ ceph osd tier add ecpool hot-storage
$ ceph osd tier cache-mode hot-storage writeback
$ ceph osd tier set-overlay ecpool hot-storage

will place the hot-storage pool as tier of ecpool in writeback mode so that every write and read to the ecpool are actually using the hot-storage and benefit from its flexibility and speed.

It is not possible to create an RBD image on an erasure coded pool because it requires partial writes. It is however possible to create an RBD image on an erasure coded pools when a replicated pool tier set a cache tier:

$ rbd --pool ecpool create --size 10 myvolume
_______________________________________________________________________________

This documentation needs to clearly state that cache tiering is a tech-review feature and will not be supported in production.