Improving Restore Speed for Backup Systems that Use Inline Chunk-Based Deduplication

Slow restoration due to chunk fragmentation is a serious problem facing inline chunk-based data deduplication systems: restore speeds for the most recent backup can drop orders of magnitude over the lifetime of a system. We study three techniques—increasing cache size, container capping, and using a forward assembly area—for alleviating this problem. Container capping is an ingest-time operation that reduces chunk fragmentation at the cost of forfeiting some deduplication, while using a forward assembly area is a new restore-time caching and prefetching technique that exploits the perfect knowledge of future chunk accesses available when restoring a backup to reduce the amount of RAM required for a given level of caching at restore time.

We show that using a larger cache per stream—we see continuing benefits even up to 8 GB—can produce up to a 5–16X improvement, that giving up as little as 8% deduplication with capping can yield a 2–6X improvement, and that using a forward assembly area is strictly superior to LRU, able to yield a 2–4X improvement while holding the RAM budget constant.