I think there's a bit of a relationship with the age of the instance type. Whether that's supply chain, different policies around spare capacity, or even customer demand I don't know.<p>My specific area only has need for a relatively small number of nodes - and its been a while since we've had much trouble getting i3en.* instances in Sydney.<p>The new i4i's though? Kinda feels like they've, only got 3 hosts.. or 2 if Greg's off sick and can't switch on the one under his desk.
We recently moved to attribute based instance selection for our web traffic. It turns out that there's a ton of capacity in random instance classes at various times. Sometimes it's c6a.metal with 192 vCPU. Sometimes it's i3.metal with 64 vCPU.<p>We've been able to get the capacity we need much more readily with this configuration. (everything is spot btw)
i suspect its more likely that there are pools for c5a.large, xlarge, etc. and that they just aren't prioritizing as much buffer/liquidity in the large instance type pools. with chip shortages it may be worth it to them to prioritize more common workloads or those which they can get a consistent percentage of a host rented vs. all-or-nothing.