Git 2.55 reftable backend writes 10,000 refs in 40ms, but concurrency locks remain a concern

Git 2.55.0, released on 29 June 2026, ships the reftable ref storage format, which stores all references in compact binary-searchable table files instead of one file per branch or tag. Benchmarks show reftable creates 10,000 refs in roughly 40ms compared to 436–650ms with the legacy files backend, and also cuts disk usage dramatically — 272KB versus 40MB at 10,000 refs. However, a developer testing concurrent writes found that 94 of 150 simultaneous git update-ref processes failed with lock errors on reftable, while the old files backend handled all 150 without failure. Read performance gains were more modest, with git for-each-ref only about 1.3–1.4x faster, and claimed fetch/push speedups of 18–22x could not be reproduced in local file-transport tests. Git 3.0 is expected to make reftable the default format for new repositories, but the concurrency limitation is a trade-off that official benchmarks have largely not highlighted.