Loosely Time-Synchronized Snapshots in Object-Based File Systems

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A file system snapshot is a stable image of all files and directories in a well-defined state. Local file systems offer point-in-time consistency of snapshots, which guarantees that all files are frozen in a state in which they were at the same point in time. However, this cannot be achieved in a distributed file system without global clocks or synchronous snapshot operations. The authors present an algorithm for distributed file system snapshots that overcomes this problem by relaxing the point-in-time consistency of local file system snapshots to a time span-based consistency. Built on loosely synchronized server clocks, it makes snapshots available within milliseconds, without any kind of locking or synchronization.