The difference between ceph weight and reweight

Using the Ceph OSD Tree command to view the Ceph cluster, you will find weight and reweight two values

Weight weight and disk capacity, general 1T, value is 1.000, 500G is 0.5

It is related to the capacity of the disk and does not change due to reduced disk available space

It can be set with the following command

Ceph OSD Crush Reweight

The reweight is a value between 0 and 1 and can be set with the following command,

Ceph OSD Reweight

The weight value does not change when reweight changes. It affects the mapping relationship of the PG to the OSD. The purpose of reweight parameter, because of the random distribution of Ceph's crush algorithm, is the data equalization in probability statistic sense, when the small-scale cluster, PG data is relatively few, it will produce some uneven situation, by adjusting the reweight parameters to achieve the data equalization.

Note that this parameter is not persisted, and when the OSD is out, the value of Reweight is 0, and when the OSD is back up, the value reverts to 1, and the previously modified value is not maintained