Redundancy Does Not Imply Fault Tolerance [pdf]

"Modern distributed systems do not consistently use redundancy to recover from file-system faults: a single file-system fault can cause catastrophic outcomes such as data loss, corruption, and unavailability."

Systems studied were Redis, ZooKeeper, Cassandra, Kafka, RethinkDB, MongoDB, LogCabin, and CockroachDB. Serious errors in file system fault recovery were found in all systems, resulting in crashes or reduced redundancy, and some systems even spread corruption to other replicas.