When you write to a file, you generally don't write to physical storage. Instead the writes get buffered in memory and written to physical storage in batches. This substantially improves performance but creates a risk: If there is some sort of outage before the data is flushed to disk, you might lose data.

In order to address that risk, you can explicitly force data to be written to disk by calling fsync. Databases generally do this to ensure durability and only signal success after fsync succeeded and the data is safely stored.

So ClickHouse not calling fsync implies that it might lose data in case of a power outage or a similar event.