Make slices and scroll configurable in Elasticsearch reindex
Problem to solve
In !34069 (merged) we introduced the ability to reindex your Elasticsearch cluster from within GitLab.
When doing some benchmarking for gitlab-com/gl-infra/production#2408 (comment 383415748) we found that we could substantially increase the reindex speed by tweaking the slices parameter. Additionally in the past we've had problems with the scroll parameter defaulting to too short a time and the reindex fails as the scrolls is deleted mid way through.
In order for us to make use of the reindex option in GitLab we'll need to be able to increase both of these parameters. There won't ever be 1 correct number for all users since there are tradeoffs (eg. higher CPU when higher slices) and the larger number of slices won't be appropriate for small clusters at all.
Intended users
User experience goal
Proposal
We need some way to configure these parameters when triggering the reindex. Perhaps a couple of inputs next to the button or pop up when you click the button before submitting or some other design.