Staging reconstructs and writes every file in a plan -- one decompress, delta-chain application and hash verification each -- and on a wide repository that is tens of thousands of files for a single open or update. The work is per-file independent, and the store's read path is offset-addressed pread over an index immutable for the duration, so it parallelises without coordination.

The one thing that cannot be written concurrently is last_stage_fault, a module global. Workers keep their own and the winner is chosen by ENTRY INDEX, which is exactly the fault sequential order would have raised, so this is observationally identical to the loop it replaces including which failure gets reported. Below 64 entries it stays sequential, and a thread that fails to spawn is not an error -- its share drains off the same cursor.

Measured 2:33.5 to 2:28.8 on a 299 MB sqlite store, about 3%, with CPU never leaving 101%. So staging is NOT the dominant term there: the flatten's per-node reconstruction is, which this does not touch. Committed so it can be measured at pkgsrc scale, where the file count is 27x higher and the size distribution is different. If it does not move that number either, revert it -- it buys complexity for 3%. check-in: 1a4b74f2d5 user: dmestas tags: sub-manifest