What is primary and secondary clustering in hash?

Primary Clustering

1. Primary clustering is the tendency for a collision resolution scheme such as linear probing to create long runs of filled slots
   near the hash position of keys.
2. If the primary hash index is x, subsequent probes go to x+1,
   x+2, x+3 and so on, this results in Primary Clustering.
3. Once the primary cluster forms, the bigger the cluster gets, the
   faster it grows. And it reduces the performance.

Secondary Clustering

1. Secondary clustering is the tendency for a collision resolution scheme such as quadratic probing to create long runs of filled slots
   away from the hash position of keys.
2. If the primary hash index is x, probes go to x+1, x+4, x+9,
   x+16, x+25 and so on, this results in Secondary Clustering.
3. Secondary clustering is less severe in terms of performance hit than primary clustering, and is an
   attempt to keep clusters from forming by using Quadratic Probing.
   The idea is to probe more widely separated cells, instead of those
   adjacent to the primary hash site.