Nearfull Ratio

❓ What?

mon_osd_nearfull_ratio is the maximum / threshold percentage of disk space used before an OSD is considered as a nearfull-osd. When utilisation crosses nearfull-ratio, it can reach backfillfull-ratio and full-ratio. The default value is 0.85 or 85% of the total available space / capacity.

❔ Why?

The ratio is set as a way to alert and prevent OSDs/disks from reaching backfillfull-ratio and full-ratio.

On croit / IBM / SUSE, once nearfull-ratio is reached, pools in the cluster are marked read-only until the nearfull-ratio is increased or more storage space is added or the cluster is balanced in a way that no OSD reaches nearfull-ratio.

On ceph, pools are read-only when full-ratio is reached.

🎤 How?

It is set on the OSDMap during cluster creation with this parameter:

[global]
[...]
mon_osd_nearfull_ratio = 0.85

If the setting is to be changed after cluster creation, one can do so with:

ceph osd set-nearfull-ratio 0.85

On croit, one can do this by adjusting the sliders at Maintenance → Full Ratios (at the bottom):

👓 References

https://docs.ceph.com/en/quincy/rados/configuration/mon-config-ref/#storage-capacity