Not the person you’re responding to, but you can merge HLLs together, so if your workload was skewed, you could hash the value you’re adding to the HLL and distribute it among more keys in FoundationDB.

Additionally, depending on the write rate and the size of the data being written to the HLL, it may be worth only actually writing it out periodically and keeping a log you read at runtime of recent values.

There is a trade off between needlessly re-writing mostly unchanged data and read performance that is similar to the IO amplification trade off in log structured merge tree-derived storage engines.