I can't find any evidence showing that OLAP means it is okay to lose data from unexpected shutdowns. How can you have correct analytics without a complete set of data?

> For good reason. It's not a simple matter of choosing one of two options. The choice has consequences: performance.

It is a simple matter though. They can choose to sacrifice performance for data durability which I suspect would not be impacted very much since clickhouse acts like an append log. It just seems that Yandex doesn't care much for durability since they are just using the database to store people's web traffic. They wouldn't care if some of that data is lost so they don't use fsync.