From https://wikitech.wikimedia.org/wiki/Switch_Datacenter#Days_in_advance_preparation

• There is no ongoing long-running maintenance that affects database availability or lag (schema changes, upgrades, hardware issues, etc.). Depool those servers that are not ready.
• Replication is flowing from eqiad -> codfw and from codfw -> eqiad (sometimes replication gets stopped in the passive -> active direction to facilitate maintenance)
• All database servers have its buffer pool filled up. This is taken care automatically with the automatic buffer pool warmup functionality. For sanity checks, some sample load could be sent to the MediaWiki application server to check requests happen as quickly as in the active datacenter.
• Make absolutely sure that parsercache replication is working from the active to the passive DC. This is important