VReplication: tracking rows_copied

[shlomi-noach]

Description

_vt.vreplication has new column, rows_copied, which is updated periodically during vreplication work, to reflect the actual number of rows written on the target table.

The value is written together with time_updated, so it does not get transactionally synced with each copy iteration, it follows closely though, and makes a good estimate.

This will be later used by Online DDL to track Vreplication-based migration progress & ETA.

Related Issue(s)

• Replaces VReplication: tracking rows_copied #7963
• Tracking: Online DDL: tracking issue #6926

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[shlomi-noach]

Good to review!

[rohit-nayak-ps]

LGTM

Just noting here that after the copy phase the rows_copied will never change.

[shlomi-noach]

@rohit-nayak-ps currently - that's the objective. Just count the number of rows copied by vcopier. There is no adjustment by vplayer's work. That'll come slightly later, and I see some larger things at play, I'll write more tomorrow. So anyway, leaving this to a different PR.

While this PR is good, I do notice that on a very long running vreplication stream, and with near zero traffic, GenerateUpdatePos either doesn't get called or is not applied. I see 25 minutes go by without an update to time_updated, nor transaction_timestamp, and thereby obviously not to rows_copied.

That's an unexpected behavior. It also relates to what I'll write more about tomorrow. So this is another thing to iterate on: I'd like to see rows_copied on the fly and frequently (at least once per minute).