s6 should be the first section to be upgraded entirely to 10.4 and Buster.
Steps to upgrade:
- Sanitarium hosts on both DCs (already done https://phabricator.wikimedia.org/T275112 https://phabricator.wikimedia.org/T272008)
- Sanitarium masters on both DCs (https://phabricator.wikimedia.org/T280492) - ensure sanitarium replicates from a 10.4 host.
- Available backup source on the standby DC
- Candidate master on the standby DC
- Master on the standby DC
- Switchover backup generation standby DC https://gerrit.wikimedia.org/r/c/operations/puppet/+/681621
- Candidate master on the primary DC
- Available backup source on the primary DC
- Switchover on the primary DC to promote a Buster+10.4 host to master: T282124
- Switchover backup generation Primary DC https://gerrit.wikimedia.org/r/c/operations/puppet/+/685494
- Upgrade the old master and make it a candidate master.
- Cleanup (remove) old backup sources from both DCs https://gerrit.wikimedia.org/r/685726 https://gerrit.wikimedia.org/r/692341
Please read the doc about procedure for more details.