You need to set a memory target for your OSDs, or they will gobble up as much as possible for their cache. In Rook you do that by specifying the resource limits on your CephCluster[0].

Sadly, Rook enforces some pretty high minimum values (2GiB per OSD, 1 GiB per mon), which is fine for production but can make homelab experimentation annoying.

[0]: https://rook.io/docs/rook/v1.1/ceph-cluster-crd.html#cluster...