Revision history [back]

Increasing the number of OSDs in Ceph will result in smaller Placement Groups (PGs) than expected because the number of PGs is determined by a formula that takes the total number of objects and the number of OSDs into account. The formula for calculating the number of PGs is:

PGs = (total objects) / (num OSDs x PGs per OSD)

When the number of OSDs is increased, the denominator in the formula becomes larger, causing the overall number of PGs to decrease. This is because the algorithm aims to balance the number of PGs per OSD, and with more OSDs available, there is less need for extra PGs for redundancy. However, having fewer PGs means that each PG is responsible for more objects, potentially leading to performance issues if the cluster is not properly balanced. It is important to consider the optimal number of OSDs and PGs for your specific use case and workload to ensure the best performance and scalability.