feat: support dynamically change max-subcompactions using config set command

[cheniujh]

支持使用config set 动态修改max-subcompactions参数。

应用场景：
当发现Rocksdb实例出现了由L0层文件达到上限而引起的write stall（L0层compaction速度跟不上flush速度）, 可以尝试将max-subcompactions改大，另外该值不占用max-background-jobs的份额，他会直接启动新的lwp来将一个大的L0层compaction分割为多个小compaction并发处理。

Summary by CodeRabbit

• New Features

  • Introduced a new configuration parameter, max-subcompactions, allowing users to manage subcompaction settings.
  • Enhanced configurability of RocksDB options with the addition of max_subcompactions in the database options.

• Bug Fixes

  • Improved error handling for invalid inputs related to the new configuration parameter.

[coderabbitai]

Walkthrough

The pull request introduces a new configuration parameter, max-subcompactions, to the ConfigCmd class in src/pika_admin.cc. This addition includes updates to the ConfigGet and ConfigSet methods to handle the new parameter, ensuring it is validated and integrated into the existing configuration management. Additionally, a corresponding entry for max_subcompactions is added in the options_helper.h file to map it to the RocksDB options, enhancing configurability.

Changes

File Path	Change Summary
src/pika_admin.cc	Added max-subcompactions parameter to ConfigCmd, updated ConfigGet and ConfigSet methods to handle it, including validation and error handling.
src/storage/src/options_helper.h	Added entry for max_subcompactions in mutable_db_options_member_type_info for RocksDB options.

Possibly related PRs

• feat: a new strategy for auto compaction (ospp 2024) #2816: The changes in this PR enhance the compaction logic and storage options in the PikaServer class, which is closely related to the new max-subcompactions configuration parameter introduced in the main PR, as both involve improvements to compaction strategies and configuration management.

Suggested labels

✏️ Feature, 4.0.1

Suggested reviewers

• dingxiaoshuai123

🐰 In the burrow where changes bloom,
A new config finds its room.
Max subcompactions, a clever tweak,
In storage's heart, it starts to speak.
With options set, the data flows,
In Pika's realm, the magic grows! 🌱

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL

📥 Commits

Reviewing files that changed from the base of the PR and between 937ebde and 711f64b.

📒 Files selected for processing (2)

• src/pika_admin.cc (2 hunks)
• src/storage/src/options_helper.h (1 hunks)

🔇 Additional comments (3)

src/storage/src/options_helper.h (1)

41-41: LGTM!

The addition of max_subcompactions option mapping is consistent with other integer DB options and correctly uses the offsetof macro to get the member offset.

src/pika_admin.cc (2)

1812-1816: LGTM!

The implementation of max-subcompactions in ConfigGet follows the established pattern and correctly handles the response encoding.

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2686-2698: LGTM!

The implementation of max-subcompactions in ConfigSet is robust with proper:

• Input validation
• Error handling
• Thread-safe option update via RocksDB
• Configuration persistence

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