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Updated the limitation of Schema Cache #19039

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Nov 12, 2024
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Updated the limitation of Schema Cache #19039

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Update schema-cache.md
Frank945946 Nov 12, 2024
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Frank945946 Nov 12, 2024
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hfxsd Nov 12, 2024
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hfxsd Nov 12, 2024
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hfxsd Nov 12, 2024
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hfxsd Nov 12, 2024
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5 changes: 5 additions & 0 deletions schema-cache.md
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Expand Up @@ -36,3 +36,8 @@ summary: TiDB 对于 schema 信息采用基于 LRU 的缓存机制，在大量
- `SHOW FULL TABLES`
- `FLASHBACK`
- `ALTER TABLE ... SET TIFLASH MODE ...`
- 对于设置了 [`AUTO_INCREMENT`](/auto-increment.md) 或 [`AUTO_RANDOM`](/auto-random.md) 属性的表，如果 schema 缓存设置过小，这些表的元信息可能会在内存中被频繁地缓存和淘汰。这种频繁的缓存变动可能导致未使用完的 ID 段失效，从而引发 ID 跳变。在写入量较大的场景下，甚至可能导致 ID 段耗尽。为减少此类问题并提升系统稳定性，建议采取以下措施：

- 通过监控面板查看 schema 缓存的命中率和大小，以评估缓存设置是否合理。并适当调大 schema 缓存大小，以减少频繁的缓存淘汰。
- 将 [`AUTO_ID_CACHE`](/auto-increment.md#auto_id_cache) 设置为 `1`，以防止 ID 跳变。
- 合理设置 `AUTO_RANDOM` 的分片位和保留位，避免可分配 ID 范围过小。