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Comment by elmin | Hacker News Reader

It's a bit odd that they don't stop launching new VMs on the old hardware. That would allow people who wanted to control the transition to just stop and start their VMs.

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This was my reaction, too. Thinking it over some, I think it's likely that certain tiers of instance types are more affected than others. And it's also likely that even though AWS seems to have lots of open capacity available, they probably are operating at a pretty high percentage, so gradually idling 10% of their host hardware would probably put a pinch on availability. The average lifetime of instances may also be a factor. I have a few long-running instances going most of the time, but I do tend to start and stop dozens of instances within minutes, depending on what I'm working on. So for those use cases, it hardly matters.

And of course, Amazon has an interest in encouraging its users to build their systems in a cloud-friendly way. ie, properly designed services on AWS should not suffer from having a handful of VMs get rebooted at any time. So, from that POV, it's just good medicine to encourage the culture they built their service to accommodate.

  they probably are operating at a pretty 
  high percentage, so gradually idling 10% 
  of their host hardware would probably 
  put a pinch on availability

If they can withstand the loss of one of out of three availability zones, they must have at least 33% spare capacity in each AZ, ready for new instances to start to take up the slack from the lost AZ.

Even if they have more availability zones than the three they show to users, I would hope they would have more than 10% spare capacity!

They do have more than 3 AZs. Our account has access to 5 distinct zones and I suspect there are more than that.

That depends which region you're in. us-east has like 5 AZs but some of the small regions like Sydney have only two.

All regions have a minimum of 3 zones, but the client doesn't always see all of them. Sydney has 3 AZs, but you only get to see 2, for example.

Not all regions (see US GovCloud for example).

I doubt they have quite that much spare capacity in any AZ. In the past (2012) they seemed to be capacity constrained when a whole AZ went offline. In a situation like that, some customers would move to a different AZ quickly, and others would just wait it out. Spot Instance pricing would also jump quickly, which would free up some capacity when low-budget workloads shut down.

Capacity dedicated to failover is not spare, when you've sold failover capability to your customers.

"there is no guarantee your new instance would not also need to be rebooted during the update window."

It sounds like you could spin up new instances until you get one that doesn't require the update.

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