How to use a SSD as a cache for HDD on Ceph

ceph-volume lvm prepare --data /dev/sdb --block.db /dev/sda1 --block.wal /dev/sda2

sdb is a harddisk. sda1 and sda2 are partitions on a SSD. You dont have to make any filesystems on the drives. I used fdisk to split the SSD in two parts. If prepare succeeds you can use

ceph-volume lvm activate --all

to start up the OSD. One difficult part is the keyring for ceph-volume. ceph-volume is not using the id client.admin. Do this if ceph-volume cant login.

ceph auth get client.bootstrap-osd >/var/lib/ceph/bootstrap-osd/ceph.keyring