Issue
Say I have three options:
| Instance type | vCPU | Memory (GiB) | Bandwidth (Gbps) | Price |
|---|---|---|---|---|
| Small | A | B | C | D |
| Medium | 2A | 2B | 2C | 2D |
| Large | 4A | 4B | 4C | 4D |
What are the implications of running 4 small instances vs 2 medium vs 1 large? Is one better or are they the same?
I am very new to all of this, thanks in advance.
Solution
Your table is accurate within an Instance Type family. For example, m5.large, m5.xlarge and m5.2xlarge.
Some benefits of using multiple (smaller) instances are:
- Distribute instances across multiple Availability Zones (different data centers) to avoid potential outage time (it is rare, but can happen)
- If something goes wrong on a single instance, only a portion of your capacity is lost rather than losing all capacity
- If your application is not capable of using multiple CPUs, then running threads on separate instances will perform better than running a single thread on a larger instance
- Instances can be updated (eg installing a new version of software) individually, so only a portion of your total capacity is unavailable during the upgrade process, rather than it all being unavailable
- You can test a newer version of your software on a single instance and monitor it closely to confirm that it is behaving as expected, rather than needing to roll-out changes all at once
The down-sides of using multiple instances are:
- More overhead having to run operating systems on each instance (so less RAM and CPU is available to the application)
- The application needs to be designed to run across multiple instances (typically in a stateless manner)
- A Load Balancer is required to distribute the load (assuming you are running a web app)
Answered By - John Rotenstein
