While upgrading 3451 SNOs with the upgrade split across 4 CGUs (SNO Counts per CGU - 1000, 1000, 1000, 451) the last CGU encountered a condition where none of the 451 SNOs performed the upgrade because all showed a status of BackupTimeout.  Later inspection of the SNOs revealed all actually completed the backup job.  Based on the timestamps and logs it seems depending on a CGUs timeout and the schedule of when a CGU is enabled, you may hit a condition where TALM is reconciling for a lengthy period of time on a previous CGU which prevents the next CGU from performing its backup.

ACM - 2.7.0-DOWNSTREAM-2023-01-26-20-15-10
Hub OCP 4.12.1
SNO OCP 4.11.24 upgrading to 4.12.1

Depends on if the number of clusters to upgrade is large enough per CGU and when CGUs are enabled for upgrade, whether or not backup is enabled as well.

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