Backfilling an OSD

When you add Ceph OSDs to a cluster or remove them from the cluster, the CRUSH algorithm rebalances the cluster by moving placement groups to or from Ceph OSDs to restore the balance. The process of migrating placement groups and the objects they contain can reduce the cluster operational performance considerably. To maintain operational performance, Ceph performs this migration with the backfill process, which allows Ceph to set backfill operations to a lower priority than requests to read or write data.