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The AWS documentation states that dedicated instances are run on hardware that’s dedicated to a single customer.

I can’t believe this means that a single dedicated instance will get a whole physical host to itself - surely that would be uneconomical for AWS, as dedicated instances only cost a small percentage more than regular instances.

So if my disbelief is true, it must mean that dedicated instances have some portion of a host dedicated to them. But what portion exactly? Do you get a physical CPU rather than a vCPU, and is that all? Does the rest of the physical host (motherboard, network interface, power supply, other bits) not count as hardware on which an instance is run?

jl6
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  • A given host is shared with other dedicated and non-dedicated instances from the same AWS account. – jarmod Jul 07 '23 at 19:02
  • So if I have just one dedicated instance and nothing else, then I occupy a whole host? That surely can’t be right. – jl6 Jul 07 '23 at 22:19
  • If I read the pricing pages correctly, you'd be paying an additional $2 per hour to do that in addition to the on-demand price for a dedicated instance (which appears to be 110% of the on-demand, non-dedicated instance price). So your single m4.xlarge, for example, would cost $2.22 per hour instead of $0.20 per hour. – jarmod Jul 07 '23 at 22:54
  • Yes. If you have just one dedicated instance and nothing else, then you occupy the whole host. If you have just one dedicated instance but you also have other dedicated or non-dedicated instance in the same AZ (and, I believe, of the same Instance Type), then those instances might run on the same host too. However, the host will not run instances for a different AWS Account. – John Rotenstein Jul 07 '23 at 23:34
  • @JohnRotenstein that is not correct - the host will also run instances from other AWS account, the entire host is not dedicated to the account. – gshpychka Jul 10 '23 at 11:25
  • @gshpychka From [Dedicated Instances - Amazon Elastic Compute Cloud](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/dedicated-instance.html): _"Dedicated Instances are EC2 instances that run on hardware that's dedicated to a single customer. Dedicated Instances that belong to different AWS accounts are physically isolated at a hardware level, even if those accounts are linked to a single payer account. However, Dedicated Instances might share hardware with other instances from the same AWS account that are not Dedicated Instances."_ That means the account gets the whole host computer. – John Rotenstein Jul 10 '23 at 12:38
  • @JohnRotenstein Hm, that's not how I read it. I took it to mean that part of the hardware of the host that runs your instance is dedicated to you. Not the entire host's hardware, since there is no "a1 medium host", the host that a1.medium runs on is actually a1.metal. That's what allows a dedicated a1.medium to be 15x cheaper than a1.metal. – gshpychka Jul 10 '23 at 13:23
  • I have removed my answer for now, would be curious to confirm your interpretation with AWS support. – gshpychka Jul 10 '23 at 13:38
  • @JohnRotenstein "physically isolated at a hardware level" Since it doesn't say explicitly that the account gets the whole host, I am skeptical that's what it means, especially given the huge cost differential that gshpychka points out. – jl6 Jul 11 '23 at 08:37
  • Also see [Why is dedicated host more expensive than dedicated instance?](https://stackoverflow.com/questions/64309679/aws-dedicated-host-vs-dedicated-instance-why-the-first-is-more-expensive-than) – jarmod Jul 11 '23 at 15:16
  • Yeah, I guess the $2 fee (per region) makes this make sense financially. If you have at least 1 dedicated instance, you pay $2/hr extra, which is huge. If you have two or more, then there are no extra fees, but those extra instances will likely run on the same host. – gshpychka Jul 11 '23 at 16:07
  • @gshpychka: Does it though? An x1e.xlarge dedicated _instance_ costs $0.9170/hr on demand in us-east-1. That's $2.9170/hr including the $2/hr per-region fee. But an x1e dedicated _host_ costs $29.357/hr. Is Amazon really just swallowing the difference? – jl6 Jul 12 '23 at 10:09
  • It works out for them on average because people who need dedicated instances usually need more than one. I don't think they're "swallowing" the cost - they're also giving you hosts that otherwise would be idle, generating $0 – gshpychka Jul 12 '23 at 10:21

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From Dedicated Instances - Amazon Elastic Compute Cloud:

By default, EC2 instances run on shared tenancy hardware. Dedicated Instances are EC2 instances that run on hardware that's dedicated to a single customer. Dedicated Instances that belong to different AWS accounts are physically isolated at a hardware level, even if those accounts are linked to a single payer account. However, Dedicated Instances might share hardware with other instances from the same AWS account that are not Dedicated Instances.

That means the account gets the whole host computer. It is not shared with any other AWS account.

John Rotenstein
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  • How does it make economic sense for Amazon? An x1e.xlarge dedicated instance costs $0.9170/hr on demand in us-east-1. That's $2.9170/hr including the $2/hr per-region fee. But an x1e dedicated host costs $29.357/hr. Is Amazon really just swallowing the difference? – jl6 Jul 12 '23 at 10:10
  • You'll need to ask AWS! Note that multiple dedicated and non-dedicated instances from the same AWS Account can still run on that host, so it isn't necessarily always just running one instance. – John Rotenstein Jul 12 '23 at 10:43