logSink: revert defaultBufferChanSize, add a trigger to flush logSink.units when chennal is full

[dengqee]

What problem does this PR solve?

1259 2424

What is changed and how it works?

Recover defaultBufferChanSize to 20480 which increased by 1469.

Add a criterion for refreshing logSink.units, which sendEvents.Load() == defaultBufferChanSize, i.e. units chennal is full.

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch

Release note

fix a bug where synchronizing large tables to cdclog failed.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• amyangfei
• overvenus

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[amyangfei]

Thanks for your contribution!

By the way, we lack unit tests in cdc/sink/cdclog, that is quite a problem

[overvenus]

LGTM, thank you!

[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: 6ef8d7a

[amyangfei]

/run-integration-tests
/run-kafka-tests

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2444.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2445.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2446.