File systems like ZFS, WAFL, and BTRFS use a copy-on-write transactional model to implement updates, crash recovery, snapshots, and clones. The resulting disk layout is one where there could be many physical pointers to a single block. This allows quick lookup when accessing a block from all the snapshots it belongs to.
When performing defragmentation, blocks are moved, requiring the update of all incoming pointers. A reverse lookup capability is needed, allowing the quick location of all pointers to a particular block. One solution is to track back-references. However, this adds an additional meta-data structure that needs to be kept up-to-date, taking up disk space and memory.
This document suggests a different approach, applying ideas from the realm of programming language garbage collection to file-system defragmentation.
By: Ohad Rodeh
Published in: RJ10465 REVISED in 2010
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