Current situation of Doris is that the cluster is balanced, but the disks of a backend may be unbalanced.
for example, backend A have two disks: disk1 and disk2, disk1's usage is 98%, but disk2's usage is only 40%.
disk1 is unable to take more data, therefore only one disk of backend A can take new data,
the available write throughput of backend A is only half of its ability, and we can not resolve this through load or
partition rebalance now.
So we introduce disk rebalancer, disk rebalancer is different from other rebalancer(load or partition)
which take care of cluster-wide data balancing. it takes care about backend-wide data balancing.
[For more details see #8550](https://github.com/apache/incubator-doris/issues/8550)
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