Why there are lots of reading operations to disk when load data using YCSB


I’m a fresh man to Couchbase. I have a question. When I tested the Couchbase using YCSB, I found a phenomenon. At the loading step, there was no reading operations of the disk at the beginning, however, there were lots of reading operations as the loading went on. Why there were so many reading operations during the loading step which just insert the item to the datebase?