Data Rebalancing

When an OSD is added to the Ceph storage cluster, the cluster map needs to be updated by using the new OSD. Calculating PG IDs will alter the cluster map, which consequently changes the object placement because the input conditions for PG ID calculation have changed. The following figure shows the rebalancing process (for reference only; the actual process is subject to the specific conditions). Some PGs instead of all PGs are migrated from the existing OSDs (OSD 1 and OSD 2) to the new OSD (OSD 3). The CRUSH algorithm is stable even during rebalancing. Most PGs retain their initial configurations, and each OSD has spare space. Therefore, load on the new OSD does not increase sharply after rebalancing is complete.

Figure 1 Rebalancing process