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OneFS file system journal, which stores information about changes to the file system, is designed to enable fast consistent recoveries after system failures or crashes. The file system replays the journal entries after a node or cluster recovers from an outage.
Each EC2 cluster node contains local NVMe-based SSD storage, which is physically connected to the host server. This is also known as EC2 instance store. OneFS leverages one of the local NVMe-based SSD as the journal target storage for protecting uncommitted writes to the file system. When a node boots up, it checks its journal and selectively replays transactions to disk where the journaling system deems it necessary.