tikv-configuration-file: update flow-control override behavior for v8.5.5 (#22296)

[ti-chi-bot]

This is an automated cherry-pick of #22296

What is changed, added or deleted? (Required)

ref tikv/tikv#18994

Update documentation for storage.flow-control.l0-files-threshold and storage.flow-control.soft-pending-compaction-bytes-limit to reflect the behavior change introduced in TiKV PR #18994.

• v8.5.4 and earlier: flow-control config unconditionally overrides the corresponding RocksDB config.
• From v8.5.5: flow-control config only overrides when it is smaller than the RocksDB config, to avoid weakening RocksDB's compaction acceleration mechanism when increasing flow-control thresholds.

Also update the corresponding RocksDB config items (level0-slowdown-writes-trigger, soft-pending-compaction-bytes-limit) with the same version-specific behavior notes.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• Related TiKV PR: flow-control: fix compaction slowdown caused by increased flow-control thresholds. (#18710) tikv/tikv#18994

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the TiKV configuration documentation to accurately describe the revised interaction between TiKV's flow control parameters and RocksDB's internal compaction thresholds. It clarifies a critical behavior change introduced in TiKV v8.5.5, where flow control settings now conditionally override RocksDB settings to prevent unintended performance degradation.

Highlights

• Updated Flow Control Override Logic: The documentation now reflects a significant change in how TiKV's flow control configurations (l0-files-threshold, soft-pending-compaction-bytes-limit) interact with and override corresponding RocksDB settings.
• Version-Specific Behavior: For TiKV versions v8.5.4 and earlier, flow control settings unconditionally overrode RocksDB settings. From v8.5.5 onwards, the override only occurs if the flow control value is smaller than the corresponding RocksDB value.
• Performance Preservation: This new conditional override mechanism is designed to prevent weakening RocksDB's compaction acceleration mechanism, especially when flow control thresholds are increased.
• RocksDB Configuration Updates: The documentation for related RocksDB configuration items (level0-slowdown-writes-trigger, soft-pending-compaction-bytes-limit) has also been updated to detail this version-specific override behavior.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for storage.flow-control.l0-files-threshold and storage.flow-control.soft-pending-compaction-bytes-limit to reflect a behavior change starting from TiKV v8.5.5. The changes correctly centralize the detailed override logic in the corresponding RocksDB configuration sections and add version-specific notes. My review focuses on improving the clarity of the new explanations. I've provided two suggestions to rephrase sentences that could be confusing, making the behavior easier to understand for users.

[gemini-code-assist]

To improve clarity, I suggest rephrasing this sentence to explain what the effective value will be. This avoids a potentially confusing self-reference and makes the behavior easier for users to understand.

Suggested change

+ Starting from v8.5.5 and v9.0.0: when the flow control mechanism is enabled ([`storage.flow-control.enable`](/tikv-configuration-file.md#enable) is `true`), the value of this configuration item is overridden by [`storage.flow-control.l0-files-threshold`](/tikv-configuration-file.md#l0-files-threshold) only when its value is greater than `storage.flow-control.l0-files-threshold`. This behavior prevents weakening RocksDB's compaction acceleration mechanism when you increase the flow control threshold.

+ Starting from v8.5.5 and v9.0.0: when the flow control mechanism is enabled ([`storage.flow-control.enable`](/tikv-configuration-file.md#enable) is `true`), the effective threshold for write stall is the smaller value between this configuration item and [`storage.flow-control.l0-files-threshold`](/tikv-configuration-file.md#l0-files-threshold). This behavior prevents weakening RocksDB's compaction acceleration mechanism when you increase the flow control threshold.

References

1. The documentation should be clear, simple, and easy for users to understand. If something might be confusing, it should be reworded. ^(link)

[gemini-code-assist]

To improve clarity, especially since the two parameter names are very similar, I suggest rephrasing this sentence to explain what the effective value will be. This avoids a potentially confusing self-reference and makes the behavior easier for users to understand.

Suggested change

+ Starting from v8.5.5 and v9.0.0: when the flow control mechanism is enabled ([`storage.flow-control.enable`](/tikv-configuration-file.md#enable) is `true`), this configuration item is overridden by [`storage.flow-control.soft-pending-compaction-bytes-limit`](/tikv-configuration-file.md#soft-pending-compaction-bytes-limit) only when its value is greater than `storage.flow-control.soft-pending-compaction-bytes-limit`. This behavior prevents weakening RocksDB's compaction acceleration mechanism when you increase the flow control threshold.

+ Starting from v8.5.5 and v9.0.0: when the flow control mechanism is enabled ([`storage.flow-control.enable`](/tikv-configuration-file.md#enable) is `true`), the effective soft limit for pending compaction bytes is the smaller value between this configuration item and [`storage.flow-control.soft-pending-compaction-bytes-limit`](/tikv-configuration-file.md#soft-pending-compaction-bytes-limit). This behavior prevents weakening RocksDB's compaction acceleration mechanism when you increase the flow control threshold.

References

1. The documentation should be clear, simple, and easy for users to understand. If something might be confusing, it should be reworded. ^(link)