# Improving storage efficiency in Magic Pocket, our immutable blob store

[Dropbox](https://yomu.fyi/company/dropbox) · Ilya Yakovlev,Andrew Cheung,Binoy Dash,Simran Jumani,Dmitriy Meyerzon,Mark Breitenbach,Ishan Mishra,Kazuaki Okumura,Mike White,Kevin Altschuler,Facundo Agriel · Apr 2, 2026

## Summary

After introducing a direct erasure-coding service, Dropbox encountered severe storage fragmentation in its immutable blob store, Magic Pocket, due to a large number of under-filled volumes. Because their legacy compaction strategy was only designed to top off near-full volumes, the team created new multi-strategy compaction methods to aggressively consolidate sparse volumes. These approaches leverage dynamic programming and streaming pipelines to rapidly reclaim unused disk capacity at exabyte scale.

## Takeaways

- In immutable blob stores, deleted data is not freed immediately and requires active compaction to rewrite live blobs into new volumes and retire fragmented ones.
- Traditional host-and-donor compaction (L1) works well in steady state but struggles when a system accumulates a long tail of severely under-filled volumes.
- Dropbox introduced L2 (a dynamic programming packing algorithm) and L3 (a continuous streaming re-encoding pipeline) to consolidate sparse volumes two to three times faster than before.

**Tags:** [Architecture](https://yomu.fyi/topic/architecture), [Performance](https://yomu.fyi/topic/performance), [Reliability](https://yomu.fyi/topic/reliability), [Scalability](https://yomu.fyi/topic/scalability)

[Read original post](https://dropbox.tech/infrastructure/improving-storage-efficiency-in-magic-pocket-our-immutable-blob-store)