docs: add memory unit example for tidb_server_memory_limit (#23587)

[ti-chi-bot]

This is an automated cherry-pick of #23587

What is changed, added or deleted? (Required)

• Add a 90GiB example to the tidb_server_memory_limit description in system-variables.md.
• Clarify the documented unit-format example without changing the supported formats.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 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)
• v5.3 (TiDB 5.3 versions)

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

• This PR is translated from: docs: add memory unit example for tidb_server_memory_limit docs-cn#21893
• Other reference link(s): N/A

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

Summary by CodeRabbit

• Documentation
  • Updated memory-size configuration guidance to support decimal units (KB, MB, GB, TB) alongside binary units.
  • Added a 90GiB usage example.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: d47d6b2f-27ce-4056-98b7-c1e4e26df9ed

📥 Commits

Reviewing files that changed from the base of the PR and between 0769740 and f6ac055.

📒 Files selected for processing (1)

• system-variables.md

Included review availability: Your plan provides up to 4 included reviews per hour; 1 remains after this review.

---

📝 Walkthrough

Walkthrough

The documentation for two TiDB system variables now includes decimal memory units alongside binary units. It also adds 90GiB and 130MiB examples.

Changes

Memory-size documentation

Layer / File(s)	Summary
Document supported memory-size formats system-variables.md	The descriptions for tidb_server_memory_limit and tidb_server_memory_limit_sess_min_size now list decimal units and include updated examples.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to f6ac0

This documentation-only change adds a memory-unit example without changing supported behavior, so no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: qiancai

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation change and the added memory-unit example. It is concise and related to the main change.
Description check	✅ Passed	The description explains the documentation change, identifies the affected master version, and provides the related pull request link. The AI agent involvement section is omitted, but the required inf…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description explains the documentation change, identifies the affected master version, and provides the related pull request link. The AI agent involvement section is omitted, but the required information is otherwise mostly complete.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [qiancai]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch master: #23675.
But this PR has conflicts, please resolve them!