IMHO this isn't the right layer for this. An admin wishing to mirror the offload device should do so (via MD or (sigh) an HBA) and present that device in the OSD spec. ymmv.
Hello Anthony! The idea behind is to keep osds (with data on HDD) running in case of meta device goes down. For sure, my test lab with md Raid1 on 2 NS within the same NVME device is just to bring the idea and it has no sense at all in real world. In prod I mean to use, for instance, nvme0n1 and nvme1n1 united into raid1. And the problem is that ceph-volume tries to get blkid, which is obviosly none on /dev/md127. That is my intention to modify the code.
Conventional wisdom has favored instead offloading fewer OSDs to each SSD to reduce write amp and the blast radius.
Indeed, but I use fast SSD devices for wal and db. BTW, don't you like HBA's? Cuz you did sigh when mentioned HBA?:) Why? Cuz of price?