Ceph: How to place a pool on specific OSD?

I have a Ceph cluster of 66 OSD with a data_pool and a metadata_pool.

I would like to place the metadata_pool on 3 specific OSD which are having SSDs, since all other 63 OSD having older disks.

How can I force Ceph to place the metadata_pool on specific OSD?

Thanks by advance.

You need a special crush rule for your pool that will define which type of storage is to be used. There is a nice answer in the proxmox forum.

It boils down to this:

Ceph knows which drive is a HDD or SDD. This information in turn can be used to create a crush rule, that will place PGs only on that type of device.

The default rule coming with ceph is the replicated_rule:

# rules
rule replicated_rule {
    id 0
    type replicated
    min_size 1
    max_size 10
    step take default
    step chooseleaf firstn 0 type host
    step emit
}

So if your ceph cluster contains both types of storage devices you can create the new crush rules with:

$ ceph osd crush rule create-replicated replicated_hdd default host hdd
$ ceph osd crush rule create-replicated replicated_ssd default host ssd

The newly created rule will look nearly the same. This is the hdd rule:

rule replicated_hdd {
    id 1
    type replicated
    min_size 1
    max_size 10
    step take default class hdd
    step chooseleaf firstn 0 type host
    step emit
}

If your cluster does not contain either hdd or ssd devices, the rule creation will fail.

After this you will be able to set the new rule to your existing pool:

$ ceph osd pool set YOUR_POOL crush_rule replicated_ssd

The cluster will enter HEALTH_WARN and move the objects to the right place on the SSDs until the cluster is HEALTHY again.

This feature was added with ceph 10.x aka Luminous.