Loosely time-synchronized snapshots in object-based file systems

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. We present an algorithm for distributed file system snapshots that overcomes this problem by relaxing the point-intime 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. Our evaluation demonstrates that enabling and accessing snapshots involves a read/write throughput penalty of no more than 1% under normal conditions.