txn: update the document about read-consistency read check ts variable #8617

cfzjywxk · 2022-03-10T10:05:06Z

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ti-chi-bot · 2022-03-10T10:05:07Z

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TomShawn · 2022-03-10T10:54:42Z

@cfzjywxk Could you please invite a technical review? Thanks~

MyonKeminta · 2022-03-16T06:47:27Z

system-variables.md

+- 悲观事务中当 `READ-COMMITTED` 隔离级别启用之后，每个事务中 `SELECT` 语句会尝试获取一个新的 `timestamp` 用于数据读取。当开启此变量之后，`SELECT` 语句会尝试使用前一个有效的 `timestamp` 进行数据读取，初始值为事务的 `start_ts`。如果整个读取过程没有遇到更新版本，则返回结果给客户端且 `SELECT` 语句执行成功。当读到过程遇到更新的版本时，如果当前 `TiDB` 尚未向客户端回复数据，则尝试重新获取一个新的 `timestamp` 重试此语句。如果已经回复过客户端数据，则 `TiDB` 会向客户端报错。回复客户端数据量受 `tidb_init_chunk_size` 和 `tidb_max_chunk_size` 控制。在使用 `READ-COMMITTED` 隔离级别且单个事务中 `SELECT` 语句较多，读写冲突较少的场景，可通过开启此变量用以避免获取全局 `timestamp` 带来的延迟和开销。
+


I suggest you only mention that it's an optimization for RC transactions that's suitable for read-heavy workloads, and put these implementation details in a supplementary information block (so that users don't have to understand it). Like this:

> **说明：** > > * 在这里解释取 ts 之类的内部原理

I think a user may not understand what it means by retrieving a timestamp or finding a newer version. It's too complex for user.

system-variables.md

TomShawn · 2022-03-24T06:04:03Z

system-variables.md

+- 适用于悲观事务 `READ-COMMITTED` 隔离级别下，读写冲突较少场景，优化事务内读语句延迟。如果读写冲突较为严重，开启此功能会增加额外开销和延迟，造成性能回退。
+
+> **说明：**
+> 悲观事务中当 `READ-COMMITTED` 隔离级别启用之后，每个事务中 `SELECT` 语句会尝试获取一个新的 timestamp 用于数据读取。当开启此变量之后，`SELECT` 语句会尝试使用前一个有效的 timestamp 进行数据读取，初始值为事务的 `start_ts`。如果整个读取过程没有遇到更新版本，则返回结果给客户端且 `SELECT` 语句执行成功。当读的过程中遇到更新的版本时，如果当前 TiDB 尚未向客户端回复数据，则尝试重新获取一个新的 timestamp 重试此语句。如果已经向客户端返回部分数据，则 TiDB 会向客户端报错。每次向客户端回复的数据量受 `tidb_init_chunk_size` 和 `tidb_max_chunk_size` 控制。在使用 `READ-COMMITTED` 隔离级别且单个事务中 `SELECT` 语句较多，读写冲突较少的场景，可通过开启此变量用以避免获取全局 timestamp 带来的延迟和开销。


这里的【整个读取过程没有遇到更新版本】中的【更新版本】是指更新的数据版本吗？

system-variables.md

TomShawn · 2022-03-24T06:36:59Z

tidb_rc_read_check_ts 较多说明以及使用限制放在变量文档里，有点塞不下的感觉，而且也不易查找。能否考虑将特性说明放在 https://docs.pingcap.com/zh/tidb/dev/transaction-isolation-levels 这里或是哪篇文档里，而变量文档只需简单介绍变量即可?

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

system-variables.md

transaction-isolation-levels.md

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

TomShawn

LGTM

transaction-isolation-levels.md

TomShawn · 2022-03-24T07:31:06Z

/remove-status LGT1
/status LGT3

system-variables.md

TomShawn · 2022-03-24T08:57:05Z

/merge

ti-chi-bot · 2022-03-24T08:57:10Z

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

Commit hash: 5d35b6c

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txn: add document for read-consistency read tso optimization pingcap/tidb#32806

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txn: update the document about read-consistency read check ts variable pingcap/docs#7928

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txn: update the document about read-consistency read check ts variable #8617

txn: update the document about read-consistency read check ts variable #8617

cfzjywxk commented Mar 10, 2022

ti-chi-bot commented Mar 10, 2022 •

edited

TomShawn commented Mar 10, 2022

MyonKeminta Mar 16, 2022

TomShawn Mar 24, 2022

TomShawn commented Mar 24, 2022

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TomShawn commented Mar 24, 2022

TomShawn commented Mar 24, 2022

ti-chi-bot commented Mar 24, 2022

		- 悲观事务中当 `READ-COMMITTED` 隔离级别启用之后，每个事务中 `SELECT` 语句会尝试获取一个新的 `timestamp` 用于数据读取。当开启此变量之后，`SELECT` 语句会尝试使用前一个有效的 `timestamp` 进行数据读取，初始值为事务的 `start_ts`。如果整个读取过程没有遇到更新版本，则返回结果给客户端且 `SELECT` 语句执行成功。当读到过程遇到更新的版本时，如果当前 `TiDB` 尚未向客户端回复数据，则尝试重新获取一个新的 `timestamp` 重试此语句。如果已经回复过客户端数据，则 `TiDB` 会向客户端报错。回复客户端数据量受 `tidb_init_chunk_size` 和 `tidb_max_chunk_size` 控制。在使用 `READ-COMMITTED` 隔离级别且单个事务中 `SELECT` 语句较多，读写冲突较少的场景，可通过开启此变量用以避免获取全局 `timestamp` 带来的延迟和开销。

txn: update the document about read-consistency read check ts variable #8617

txn: update the document about read-consistency read check ts variable #8617

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cfzjywxk commented Mar 10, 2022

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ti-chi-bot commented Mar 10, 2022 • edited

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edited