add descriptions for two system variables #9896

cfzjywxk · 2022-08-10T02:15:39Z

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ti-chi-bot · 2022-08-10T02:15:40Z

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sticnarf · 2022-08-10T03:02:08Z

system-variables.md

+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `OFF`
+- This variable controls whether the calculated commit ts would be used for async commit. By default the two phase committer would request a new ts from the pd server and use it as the commit ts, and linearizability is guaranteed for all the concurrent transactions. This ts fetching cost could be saved if this variable is set to 'ON' which means the calculated commit is used, the cost is that causal consistency could not be guaranteed but not linearizability, more details could be referenced in this [document](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/). If the usage scenario requires only causal consistency, this vairable could be set to 'ON' to improve performance.


Perhaps the description is not very accurate. When tidb_guarantee_linearizability is ON, the new ts from tso may not be the commit ts. It can still be smaller than the max read ts.

So, the more accurate way is to say whether a new ts from the pd server is used to calculate the final commit ts.

sticnarf · 2022-08-10T03:03:21Z

system-variables.md

+- Scope: SESSION
+- Persists to cluster: No
+- Type: String
+- This variable is used to get the last transaction info within the current session. The information includes:


Shall we put it clearly that it's a read-only system variable?

zyguan

LGTM

sticnarf

LGTM

cfzjywxk · 2022-08-10T09:59:11Z

@songrijie PTAL

TomShawn · 2022-08-11T11:02:21Z

system-variables.md

+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `OFF`
+- This variable controls the way commit ts is calculated for async commit. By default the two phase committer would request a new ts from the pd server and use it to calculate the final commit ts, and linearizability is then guaranteed for all the concurrent transactions. This ts fetching from the pd server could be saved if this variable is set to 'ON', the cost is that only causal consistency could be guaranteed but not linearizability, more details could be referenced in this [document](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/). If the usage scenario requires only causal consistency, this vairable could be set to 'ON' to improve performance.


If we set this variable to ON, is the new TS fetched from PD used as the final commit TS?

Suggested change

- This variable controls the way commit ts is calculated for async commit. By default the two phase committer would request a new ts from the pd server and use it to calculate the final commit ts, and linearizability is then guaranteed for all the concurrent transactions. This ts fetching from the pd server could be saved if this variable is set to 'ON', the cost is that only causal consistency could be guaranteed but not linearizability, more details could be referenced in this [document](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/). If the usage scenario requires only causal consistency, this vairable could be set to 'ON' to improve performance.

- This variable controls the way commit TS is calculated for async commit. By default (with the `OFF` value), the two-phase commit requests a new TS from the PD server and uses the TS to calculate the final commit TS. In this situation, linearizability is guaranteed for all the concurrent transactions.

- If you set this variable to `ON`, the TS fetched from the PD server is saved, with the cost that only causal consistency is guaranteed but not linearizability. For more details, see the blog post [Async Commit, the Accelerator for Transaction Commit in TiDB 5.0](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/).

+ For scenarios that require only causal consistency, you can set this variable to `ON` to improve performance.

The new TS fetched from pd would be used to calculate the final commit TS, it may or may not be the final commit TS.

TomShawn · 2022-08-11T11:03:14Z

system-variables.md

+- Scope: SESSION
+- Persists to cluster: No
+- Type: String
+- This variable is used to get the last transaction info within the current session, it's a read-only variable. The transaction information includes:


So users can do nothing with it, right?

Suggested change

- This variable is used to get the last transaction info within the current session, it's a read-only variable. The transaction information includes:

- This variable is used to get the last transaction information within the current session. it is a read-only variable. The transaction information includes:

system-variables.md

TomShawn · 2022-08-12T09:12:13Z

/hold

TomShawn · 2022-08-12T09:16:42Z

/merge

ti-chi-bot · 2022-08-12T09:16:45Z

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

Commit hash: dea3fbc

songrijie · 2022-08-12T09:28:04Z

LGTM

ti-chi-bot · 2022-08-12T10:04:40Z

In response to a cherrypick label: new pull request created: #9956.

ti-chi-bot · 2022-10-14T03:29:21Z

In response to a cherrypick label: new pull request created: #10837.

Signed-off-by: ti-chi-bot <ti-community-prow-bot@tidb.io>

Update system-variables.md

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Update system-variables.md

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cfzjywxk changed the title ~~add descriptions for the system variable tidb_guarantee_linearizability~~ add descriptions for two system variables Aug 10, 2022

cfzjywxk requested review from TomShawn, sticnarf and zyguan August 10, 2022 02:19

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Update system-variables.md

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add descriptions for two system variables (#9896) #9956

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add descriptions for two system variables (#9896) (#9956)

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Add 3 transaction related sysvar pingcap/docs-cn#10908

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Add 3 transaction related sysvar (#10908) pingcap/docs-cn#10922

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add descriptions for two system variables (#9896) #10837

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This is an automated cherry-pick of pingcap#9896

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Add 3 transaction related sysvar (#10908) pingcap/docs-cn#11637

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add descriptions for two system variables (#9896) (#10837)

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sysvar: update tidb_guarantee_linearizability information pingcap/docs-cn#12000

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add descriptions for two system variables #9896

add descriptions for two system variables #9896

cfzjywxk commented Aug 10, 2022 •

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songrijie commented Aug 12, 2022

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ti-chi-bot commented Oct 14, 2022

-- This variable controls the way commit ts is calculated for async commit. By default the two phase committer would request a new ts from the pd server and use it to calculate the final commit ts, and linearizability is then guaranteed for all the concurrent transactions. This ts fetching from the pd server could be saved if this variable is set to 'ON', the cost is that only causal consistency could be guaranteed but not linearizability, more details could be referenced in this [document](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/). If the usage scenario requires only causal consistency, this vairable could be set to 'ON' to improve performance.
+- This variable controls the way commit TS is calculated for async commit. By default (with the `OFF` value), the two-phase commit requests a new TS from the PD server and uses the TS to calculate the final commit TS. In this situation, linearizability is guaranteed for all the concurrent transactions.
+- If you set this variable to `ON`, the TS fetched from the PD server is saved, with the cost that only causal consistency is guaranteed but not linearizability. For more details, see the blog post [Async Commit, the Accelerator for Transaction Commit in TiDB 5.0](https://en.pingcap.com/blog/async-commit-the-accelerator-for-transaction-commit-in-tidb-5-0/).
++ For scenarios that require only causal consistency, you can set this variable to `ON` to improve performance.

	- This variable is used to get the last transaction info within the current session, it's a read-only variable. The transaction information includes:
	- This variable is used to get the last transaction information within the current session. it is a read-only variable. The transaction information includes:

add descriptions for two system variables #9896

add descriptions for two system variables #9896

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cfzjywxk commented Aug 10, 2022 • edited

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