f2fs-slack-space-recycling · shipping · also F2FS SSR, Slack Space Recycle

F2FS slack space recycling

F2FS slack space recycling allocates eligible free positions within an already used segment instead of requiring a fresh sequential segment.

The Linux allocation-mode declaration expands SSR as Slack Space Recycle. [5]

In the reviewed Linux v6.12/v6.16 path, __next_free_blkoff searches for a zero in the union of current-valid and checkpoint-protection maps. Current invalidity alone is insufficient. [1] [2]

With checkpointing enabled, allocation can add a protection bit before the next checkpoint; invalidation does not clear it. With checkpointing disabled, the reviewed allocation helper does not add that bit. These rules describe the bitmap guard, not whole-filesystem free capacity or a recommendation to disable checkpoints. [2]

The FAST 2015 design explains threaded logging as filling holes in used segments, with a cleaning-versus-random-write trade-off. Its historical thresholds and measurements are not current-kernel guarantees. [3]

Relations

related

F2FS, F2FS checkpoint, F2FS Segment Information Table, Host-side flash GC

Citations

1. Linux v6.12 fs/f2fs/segment.c: update_sit_entry, __next_free_blkoff and change_curseg. (accessed 2026-10-21)
2. Linux v6.16 fs/f2fs/segment.c: separate allocation/release helpers and SSR free-slot selection. (accessed 2026-10-21)
3. Lee et al. (2015). F2FS: A New File System for Flash Storage, section 2.6. (accessed 2026-10-21)
5. Linux v6.12 fs/f2fs/segment.h: SSR allocation mode and Slack Space Recycle. (accessed 2026-10-21)

Further reading

4. Hesela: F2FS invalid does not mean reusable yet; illustrative model and tests. (accessed 2026-10-21)

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