hesela.devS-FTL
S-FTL is a research flash translation layer that caches translation pages and compresses their address mappings using physical-page contiguity.
The bitmap representation records run heads and reconstructs other physical page numbers from logical offsets. A second bit lets a base run survive an intervening local rewrite. This is exact address reconstruction, not approximate learned lookup. [1]
Fewer heads need not mean fewer bytes. In a synthetic 512-entry example with four-byte heads, one-bit storage needs 76 bytes for three heads, while two-bit storage needs 136 bytes for two. These counts cover only packed bitmaps and head addresses, not total controller RAM. [2]
The model derives the crossover explicitly: a second bit adds 64 bitmap bytes for 512 entries, so it must remove at least 17 four-byte heads for a strict payload saving. Supplied run labels and excluded metadata make this a conditional calculation, not a device result. [2]
Representation also depends on history. While a page stays cached, the paper switches into bitmap form below 80% of flat size and out above 90%; equality keeps the current form. A conditional 1728-byte bitmap versus 2048-byte flat map can therefore retain either form at the same final content. This is a size-state example, not an observed SSD trace. [1] [7] A fresh cache load instead selects bitmap only below 80%, even if an earlier residency used bitmap. Holding the candidate size fixed at 1728 bytes, reloading selects the 2048-byte flat form. Equality preserves the current form only for resident-page updates, not a remembered form across eviction. [1]
The original design also gives some small compressed pages another eviction chance and buffers sparse dirty entries separately. Bitmap bytes alone do not reproduce its replacement or writeback behavior. [1]
Relations
- related
- DFTL, SSD mapping cache, FTL translation page, Global Translation Directory (DFTL), Page-level mapping (FTL), LeaFTL
Limitations
- The original performance evaluation uses trace-driven FlashSim simulations; it does not establish a ranking for current commercial SSDs. [1]
- Compression cannot certify persistence, integrity or recovery behavior. [2]
Citations
- Jiang et al. S-FTL: An Efficient Address Translation for Flash Memory by Exploiting Spatial Locality. MSST 2011, III.B and IV.
- Hesela: synthetic S-FTL bitmap accounting, method, limits and reproduction commands.
- Hesela: actual packed bytes, backward-search decoder and history-dependent size example.
Further reading
- Hesela: canonical representation model.
- Hesela: exhaustive reconstruction and accounting tests.
- Hesela: generated synthetic mappings, bitmaps and payload counts.
- Hesela: SSD mapping caches and host memory.
- Hesela: independent packed-payload verifier.
- Hesela: packed-format boundary and history-state tests.
Sources & caveats
- Primary-source synthesis plus original synthetic arithmetic. No simulator reproduction, device benchmark or independent scientific peer review is claimed.
- The model's 68-byte one-head payload is narrower than the paper's reported 70-byte example; no implementation allocation or undocumented header is inferred.