Git 2.55 reftable writes 10,000 refs in 40ms but fails under 100 concurrent writers

Git 2.55.0 (released June 29, 2026) ships the reftable ref storage backend: 10,000 refs are created in 39–42ms versus 436–650ms on the old files backend, and 50,000 refs take 1.4MB instead of 198MB. But with 100–150 simultaneous git update-ref processes, 30–63% of operations fail with "cannot lock references"; only reftable.lockTimeout=5000 fixes it, at the cost of ~1.1s per run.

• 10,000 refs: 39–42ms on reftable vs 436–650ms on the files backend
• 50,000 refs use 1.4MB in reftable vs 198MB under .git/refs/
• At 100–150 concurrent writers, 30–63% of operations fail with cannot lock references
• reftable.lockTimeout=5000 removes the errors but slows writes to 1.05–1.15s