"Because data is immutable, deletes do not immediately free up disk space. Old data stays on-disk inside storage volumes. Once a volume is closed, it is never reopened. The tradeoff is that deletes leave unused space behind, and that waste grows over time unless we actively reclaim it."
"Earlier this year, Dropbox discovered that a new 'Live Coder' service was creating severely underfilled storage volumes, sometimes using less than 5% of their capacity. This spread data across many nearly empty volumes, increasing fragmentation and storage overhead and exposing limits in the existing compaction system."
Dropbox enhanced storage efficiency in its Magic Pocket system by implementing new compaction strategies that reorganize valid data into new volumes. This process allows old, partially filled volumes to be cleared and reused. The immutable nature of stored data means that deleted files do not immediately free up disk space, leading to increased fragmentation and storage overhead. A recent service, 'Live Coder,' exacerbated this issue by creating underfilled volumes, prompting a reevaluation of the compaction system to address these inefficiencies.
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