It looks like writes are buffered in an in-memory write ahead log before being written to object storage, which means that if the writer box dies, then you lose acknowledged writes.

I've built something similar for low-cost storage of infrequently accessed data, but it uses our DBMS (MySQL) for the WAL (+ cache of hot reads), so you get proper durability guarantees.

The other cool trick to use is to use Bε-trees (a relatively recent innovation from Microsoft Research) for the object storage compaction to minimise the number of write operations needed when flushing the WAL.