Full Ratio

❓ What?

The maximum / threshold percentage of disk space usage before an OSD is considered full. This happens after usage has crossed and Nearfull Ratio. The default value is 0.95 or 95% of the total available space / capacity.

❔ Why?

When a ceph cluster approaches full ratio, it sacrifices high availability: the cluster goes read-only at this ratio. All writes that are halted will result in degraded objects (objects that have less than Pool Size (size) ideal copies). Furthermore if the cluster has min_size 1 then it results in data loss. Therefore it’s not a good production practice.

🎤 How?

Full Ratio is set during cluster creation on the OSDMap as follows:

[global]
[...]
mon_osd_full_ratio = 0.95

Afterwards, it can be changed with:

ceph osd set-full-ratio

In croit, Full Ratio can be changed by visiting Maintenance → Full Ratios (at the bottom):

👓 References

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