Btw, it should be easy adding fsync to ClickHouse. For instance VictoriaMetrics uses similar file format as ClickHouse, and it issues proper fsyncs at least every second, so it may lose only the last second of data on unclean shutdown such as OOM, hardware reset or `kill -9`. [1], [2].

[1] https://medium.com/@valyala/wal-usage-looks-broken-in-modern...

[2] https://medium.com/@valyala/how-victoriametrics-makes-instan...