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when taking a snapshot. The two time consuming operations are scanning all the filesystem bitmaps to determine which blocks are in use and scanning all the other snapshots so as to be able to expunge their blocks from the view of the current snapshot. The bitmap scanning is broken into two passes. Before suspending the filesystem all bitmaps are scanned. After the suspension, those bitmaps that changed after being scanned the first time are rescanned. Typically there are few bitmaps that need to be rescanned. The expunging of other snapshots is now done after the suspension is released by observing that we can easily identify any blocks that were allocated to them after the suspension (they will be maked as `not needing to be copied' in the just created snapshot). For all the gory details, see the ``Running fsck in the Background'' paper in the Usenix BSDCon 2002 Conference Proceedings, pages 55-64. |
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