What happens to old and new keys when the map grows? Why not copy everything at once?

When a map in Go grows, keys are gradually redistributed from the old hash table to a new, larger one. Old and new keys can coexist in both tables during the migration process.

Copying all elements at once is avoided to prevent sharp peaks in time and memory usage — redistribution occurs gradually during subsequent map operations. This helps maintain performance and prevents long-term blocking of map access.