release note about sql.txn.upgraded_iso_level.count metric

[michae2]

Michael Erickson (michae2) commented:

When upgrading from 23.2 to 24.1, cluster setting sql.txn.read_committed_isolation.enabled will be enabled by default, meaning transactions started with BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED will actually use read committed isolation instead of silently being promoted to serializable isolation. This could be considered a breaking change, because the behavior of the two isolation levels is different.

In order to help users prepare for the 23.2 to 24.1 upgrade, we've introduced a new metric in 23.2.3 called sql.txn.upgraded_iso_level.count which shows how many transactions are currently undergoing this silent promotion from read committed isolation to serializable isolation, and would be affected. (This metric shows up in the DB console under Metrics > SQL > Upgrades of SQL Transaction Isolation Level.)

We should add a release note for users about checking this metric before upgrading to 24.1. If the metric shows that there are transactions being promoted from RC to serializable, users will want to inspect those transactions and be aware of the change in isolation level in 24.1. If they would like to continue using the silent promotion to serializable isolation, they should SET CLUSTER SETTING sql.txn.read_committed_isolation.enabled = false;.

Jira Issue: DOC-9868

[michae2]

(If it helps, this metric was added by cockroachdb/cockroach#118497.)

[exalate-issue-sync]

Shannon Bradshaw (shannonbradshaw) commented:
Assigning to Matt, because this is really more of an upgrade concern.

[exalate-issue-sync]

Ryan Kuo (taroface) commented:
This was resolved with #18522 and will also be addressed in #18530 .