1491a47479
Amendment A5 and the `[M1.1]`/`[M1.2]` blocks. What they record is a result with two halves, and the value is in keeping both: The mechanism works. 934 MB reclaimed over the update run, occupancy at 1.06-1.26x its live data, and the counters that say so are in `serverStatus` rather than inferred. The ratio did not move. 1.94x delete-heavy and 2.46x update-heavy, identical to the end-of-Stage-2 binary measured with the same harness. `file / live` is a high-water mark because the data file never shrinks, and the mark is set in the first round by the one thing reclamation cannot avoid: a rebuild needs a whole second copy of the live data before the first can be freed. So ~2x is the floor of a rebuild-based design and no threshold reaches it -- rebuilding earlier lowers the garbage term and nothing else, rebuilding later raises it. The plan said in advance what to do if this happened, which was to write it down rather than tune, and to name incremental compaction through a doc-id-to-offset indirection layer as the successor. Recorded, with its cost: a second copy-on-write B+tree per collection, a second random read on point lookup, and it undoes A3. A second lever is named that the plan had not: 52% of the steady-state file is space the database owns and is not using, so returning it to the filesystem is worth more here than reclaiming harder. It needs the file never to shrink below what the fallback generation references, which is its own crash-safety pass. D7.4's 1.65x for the delete line is corrected to 1.94x, and the correction is the harness rather than a regression -- the same 1.94x comes out of the binary that predates any of this work. The old ad-hoc version sampled ids to delete blindly, which re-picks dead ones, so it deleted fewer documents than it inserted and measured a collection that was quietly growing. The update line reproduces D7.4 exactly, 2.46 against 2.47. 200-byte documents reclaim nothing, exactly as forecast, and the forecast being written down beforehand is what makes that a result instead of a disappointment. 3.93x on both binaries. Also noted, because the number invites misreading: at that document size the two index trees are comparable to the documents themselves and the file is already 2.18x before any churn -- index structure, not slab garbage.
15 KiB
15 KiB