The maximum or threshold percentage of disk space that can be used before an OSD is considered too full to backfill. This ratio is reached after crossing Nearfull Ratio. Once utilisation crosses Backfillfull Ratio, it reaches Full Ratio. The default value is 0.90 or 90% of the total available space.
The parameter is set during cluster creation in the OSDMap as:
Afterwards, it can be changed by running the following on the admin node:
On croit, it can be changed by navigating to Maintenance → Full Ratios (at the bottom):