People may start their elasticsearch cluster with very few shards. Or even start with 1 somehow.
WATCH OUT! You might be in big troubles, when your data grows much bigger.
Like slow query performance. Endless service crash with out-of-memory issues. Even worse, adding more VMs won’t help!
And the cure? You have to re-index them. But it comes with a cost, which you wish not to pay. Trust me, it hurts. Badly.