Your Jira has 25,000 active users across the globe using 100 projects with over a million issues.
Another administrator has made a change to a field configuration scheme and you now need to re-index Jira.
What would be the impact of rebuilding the index in the foreground?
Rebuilding the index in the foreground means that the process will use the main thread of the application and block other operations until it completes. Therefore, all users will be locked out of Jira until the re-indexing completes. This is necessary to ensure data consistency and accuracy during the indexing process.
If it's that large an instance, it will be mutli-node Data Center, therefore reindexing will not lock the instance
why not A I understands its not E as per below: On a multi-node Jira Data Center, you can use the Full re-index option without actually locking the instance.
Answer 'E' FYI: https://confluence.atlassian.com/adminjiraserver/search-indexing-938847710.html
The answers I chose were E