[jahands]

Problem

After every sync, the post step runs git repack --geometric=2 on the mirror. On a large mirror that repack takes about as long however little the sync fetched, and every job that commits pays it. In a full CI run of a large open-source monorepo (12.6 GiB mirror, ~220 jobs), the post step averaged 14.4s and reached 56s.

Change

Skip the repack until 50 small packs have built up, which is git's gc.autoPackLimit default. The rest of mirror maintenance is unchanged.

Results

	Before	After
Post step mean, full CI run	14.4s	3.3s
Post step max, full CI run	56s	7s
Jobs that repacked, one day of CI	all 37k committing jobs	612
Post-step time, one day of CI	~149 job-hours	~36 job-hours

• CI runs: before and after are separate runs of the repo's CI on a fork.
• One-day rows: the repo's CI jobs from 2026-11-10, replayed through a model of the mirror's snapshots. The post-step totals apply the CI runs' per-job means to those jobs.

Trade-off

Once every ~50 snapshots, the jobs that start from that snapshot still pay a repack, as every job does today. That is the 612 jobs above, 18 times in the day.

Tests

• New: maintenance skips the repack below the limit and repacks at it.
• Updated mirror-cleanup and mirror-gc-git tests for the limit.

Created with Claude Opus 5.5 in Claude Code.

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[blacksmith-staging]

Post step skips mirror repack below 50 small packs

With the rebuilt dist, the post step still skips the repack below 50 small packs (about 3s down to about 0.16s per job under 8-way load) and still rolls up at 50, leaving the same refs and objects as before.

_Merge-base

_{This PR 6217720a}

Caution

New issue found while testing

Large sync packs never reach the roll-up limit

With 55 sync packs of 10 MiB, deferral leaves 30 small packs, so every post step skips the repack and the mirror stays at 61 packs. Before, three jobs folded it to 4 packs.

_Merge-base

_{This PR 6217720a}

_{Proof on 6217720a (from ade2fa32) · mocked: Sticky-disk Git mirror (Each job gets a local copy of a public repo's bare mirror with synthetic sync pa...) · not run: Blacksmith VM agent (sticky disk gRPC) (Only exists on Blacksmith runners; the post step's commit call fails after maint...) · Did Proof get it right? Yes · No}

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