v8.5.5 br: provide a storage target option for BR restore checkpoint data (#20492)

[ti-chi-bot]

This is an automated cherry-pick of #20492

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• This PR is translated from: br: provide a storage target option for BR restore checkpoint data docs-cn#19907
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[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 significantly enhances the BR (Backup & Restore) checkpoint restore functionality by introducing an option to store checkpoint data in external storage. This change provides greater flexibility for users to manage restore checkpoints outside of the target cluster, particularly for TiDB v9.0.0 and later, where the default behavior is to store checkpoints within the downstream cluster. The documentation now clearly outlines both storage methods and their implications for snapshot and point-in-time recovery.

Highlights

• External Checkpoint Storage Option: Introduced the ability to specify an external storage target for BR restore checkpoint data using the --checkpoint-storage parameter.
• Default Checkpoint Storage Change: Clarified that starting from v9.0.0, BR stores checkpoint data in the downstream cluster by default, with the option to override this.
• Detailed External Storage Structure: Provided a detailed explanation and directory structure for how checkpoint data is organized when stored in external storage.
• Updated Restore Process Descriptions: Enhanced the descriptions of both snapshot and PITR restore phases to account for the new external storage option and updated handling of checkpoint data, including restored-ts.

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[gemini-code-assist]

Code Review

This pull request documents the new option to store BR restore checkpoint data in an external storage. The changes are clear and well-structured, splitting the implementation details into two sections based on the storage location.

I've added a few suggestions to improve consistency and technical accuracy, mainly related to using backticks for commands and parameters, and clarifying the storage location, in adherence with the repository's style guide.

One larger point of feedback is that there is significant repetition between the 'store checkpoint data in the downstream cluster' and 'store checkpoint data in the external storage' sections. To improve maintainability and readability, you could consider refactoring this to have a single 'Implementation details' section that explains the common logic and then uses sub-bullets to describe the differences for each storage method. However, I've provided comments on the current structure for more immediate improvements.

[ti-chi-bot]

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