document about 13.4 upgrade which includes a heavy-loaded hashed storage post migration task
Problem to solve
In gitlab 13.4, there is a post-migrate task that schedules projects to be moved to hashed storage. https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/db/post_migrate/20200915044225_schedule_migration_to_hashed_storage.rb. This migration task takes a longs time to finish in the background and is very resource heavy. Currently there is no mention in either upgrade instruction or transition from legacy storage to hash storage.
Further details
Proposal
It would be good to have a section taking about the potential risk for upgrading to gitlab 13.4. If customers have not moved project data to hashed storage already, it could be a potential problem because the background tasks would consume a lot of resources and cause unintentional impacts.