Just a quick one – if you remove the Search Server before upgrading to TFS 2017.2, you might see failing Git_RepositoryCodeIndexing or TFVC_RepositoryCodeIndexing jobs raising a Microsoft.VisualStudio.Services.Search.Common.FeederException: Lots of files rejected by Elasticsearch, failing this job error.
The reason why this happens is because the extension is automatically enabled on the collections, triggering these jobs.
So check your extensions after the upgrade
