**Checklist:**
NEW primary: db2127
OLD primary: db2105
[] Check configuration differences between new and old primary:
```
sudo pt-config-diff --defaults-file /root/.my.cnf h=db2105.codfw.wmnet h=db2127.codfw.wmnet
```
**Failover prep:**
[] Silence alerts on all hosts:
```
sudo cookbook sre.hosts.downtime --hours 1 -r "Codfw switchover s3 T317839" 'A:db-section-s3'
```
[] Set NEW primary with weight 0 (and depool it from API or vslow/dump groups if it is present).
```
sudo dbctl instance db2127 set-weight 0
sudo dbctl config commit -m "Set db2127 with weight 0 T317839"
```
[] Topology changes, move all replicas under NEW primary
```
sudo db-switchover --replicating-master --read-only-master --timeout=25 --only-slave-move db2105 db2127
```
[] Disable puppet on both nodes
```
sudo cumin 'db2105* or db2127*' 'disable-puppet "primary switchover T317839"'
```
[] Merge gerrit puppet change to promote NEW primary: FIXME
**Failover:**
[] Log the failover:
```
!log Starting s3 codfw failover from db2105 to db2127 - T317839
```
[] Switch primaries:
```
sudo db-switchover --replicating-master --read-only-master --skip-slave-move db2105 db2127
echo "===== db2105 (OLD)"; sudo db-mysql db2105 -e 'show slave status\G'
echo "===== db2127 (NEW)"; sudo db-mysql db2127 -e 'show slave status\G'
```
[] Promote NEW primary in dbctl, and remove read-only
```
sudo dbctl --scope codfw section s3 set-master db2127
sudo dbctl config commit -m "Promote db2127 to s3 codfw T317839"
```
[] Restart puppet on both hosts:
```
sudo cumin 'db2105* or db2127*' 'run-puppet-agent -e "primary switchover T317839"'
```
**Clean up tasks:**
[] Clean up heartbeat table(s).
```
sudo db-mysql db2127 heartbeat -e "delete from heartbeat where file like 'db2105%';"
```
[] change events for query killer:
```
events_coredb_master.sql on the new primary db2127
events_coredb_slave.sql on the new slave db2105
```
[] Update candidate primary dbctl and orchestrator notes
```
sudo dbctl instance db2105 set-candidate-master --section s3 true
sudo dbctl instance db2127 set-candidate-master --section s3 false
(dborch1001): sudo orchestrator-client -c untag -i db2127 --tag name=candidate
(dborch1001): sudo orchestrator-client -c tag -i db2105 --tag name=candidate
```
[] Check zarcillo was updated
** db-switchover should do this. If it fails, do it manually: https://phabricator.wikimedia.org/P13956
```
sudo db-mysql db1115 zarcillo -e "select * from masters where section = 's3';"
```
[] (If needed): Depool db2105 for maintenance.
```
sudo dbctl instance db2105 depool
sudo dbctl config commit -m "Depool db2105 T317839"
```
[] Change db2105 weight to mimic the previous weight db2127:
```
sudo dbctl instance db2105 edit
```
[] Apply outstanding schema changes to db2105 (if any)
[] Update/resolve this ticket.