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Problem: Issue occurs when ZooKeeper 3.5.5 is trying to load an existing 3.4 data dir in which no snapshot file has been created. This usually happens in standalone mode if txn count hasn't reached the limit of snapshot creation. This is a known issue in 3.4 versions. An extra startup check has been introduced in 3.5.5 (ZOOKEEPER-2325) to prevent a potential data inconsistency issue which makes ZooKeeper unable to start when no snapshot files present. It's a completely valid scenario in 3.4, so we have an open Jira to fix it: ZOOKEEPER-3056
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