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Do two large instances perform the same as an extra large instance? I'm not too familiar with virtualization but just wanted to know from a high level if they are more or less the same.

  • Large Instance 7.5 GB of memory, 4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each), 850 GB of local instance storage, 64-bit platform
  • Extra Large Instance 15 GB of memory, 8 EC2 Compute Units (4 virtual cores with 2 EC2 Compute Units each), 1690 GB of local instance storage, 64-bit platform

The RAM, # of virtual processors and storage is doubled for an extra large instance. However the price is doubled too. I was under the impression that Amazon would try to incentivize customers to get an extra large instance by making it's price less than two large instances.

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up vote 2 down vote accepted

Some load is not going to scale out well or only with considerable amount of overhead. So a single, more powerful machine might make more sense than two smaller ones for certain scenarios. In general, some kind of overhead is always there when using more than a single instance:

  • since you have more than one operating system instance running, the memory requirements for the kernel and necessary userspace code are multiplied
  • the same logic applies for storage - the OS data needs to be stored multiple times
  • interrupts (especially periodic ones like timers) need to be processed on all machines independently, using a multitude of CPU cycles compared to a single-instance scenario
  • if servers are working on the same data set, cached data is likely to have duplicates among several systems

But then, a scale-out approach is often taken to increase availability. Obviously, all of the above still applies there, but is accepted as the necessary price for independently running system units.

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