Update a TiDB FAQ about supported storage engine

[TomShawn]

What is changed, added or deleted? (Required)

Update a TiDB FAQ

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 (in Chinese).

• master (the latest development version)
• v6.1 (TiDB 6.1 versions)
• v6.0 (TiDB 6.0 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)

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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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lilin90
• tiancaiamao

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[lilin90]

@tiancaiamao

1. 这句话要不要保留呢？ 如果一个存储引擎是支持事务的 KV 引擎，并且能提供一个满足 TiDB 接口要求的 Client，即可接入 TiDB。
2. 未来 TiDB 版本可能不再支持 MockTiKV 这里的 未来 能否稍微具体一下？
3. 除了 TiKV 之外 处是否要提 TiFlash，即改为 除了 TiKV 和 TiFlash 之外？

[tiancaiamao]

1. 如果一个存储引擎是支持事务的 KV 引擎，并且能提供一个满足 TiDB 接口要求的 Client，即可接入 TiDB ... 这个是历史问题，最初的 tidb 代码很简单时，只要随便弄一个 kv 引擎，实现 Client 接口就可以接入 TiDB 了... 等后来 tidb 越来越复杂，需要实现复杂的事务协议，并且实现 coprocessor 的相关接口，这已经不是随便拿一个 kv 引擎很容易能实现的了
2. 我们做 mocktikv 主要是用来跑测试用。再做后来 unistore 是为了更好的性能以及一些实验性特性的验证... 等 unistore 成熟后就拿它当默认的跑测试用。当前已经没有在 MockTiKV 上面做任何修改了，MockTiKV 已经属于废弃状态... 不过还没有做清理，并且 tidb 的 -store=mocktikv 还在，未来 如果清理掉代码，这个就不支持了
3. TiFlash 其实不算一个完整的实现，它只实现了读，没有实现写。TiDB 是不能直接写入 TiFlash 的，不过可以读... 也就是 TiFlash 只实现了 Client 接口其中 coprocessor 一部分的 API，但是没实现事务部分的 API

[lilin90]

LGTM

[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 28367cb

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #9491.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #9492.