[otterley]

There are definitely cost and other considerations you have to think about when going multi-AZ.

Cross-AZ network traffic has charges associated with it. Inter-AZ network latency is higher than intra-AZ latency. And there are other limitations as well, such as EBS volumes being attachable only to an instance in the same AZ as the volume.

That said, AWS does recommend using multiple Availability Zones to improve overall availability and reduce Mean Time to Recovery (MTTR).

(I work for AWS. Opinions are my own and not necessarily those of my employer.)

[znep]

This is very true, the costs and performance impacts can be significant if your architecture isn't designed to account for it. And sometimes even if it is.

In addition, unless you can cleanly survive an AZ going down, which can take a bunch more work in some cases, then being multi-AZ can actually reduce your availability by giving more things to fail.

AZs are a powerful tool but are not a no-brainer for applications at scale that are not designed for them, it is literally spreading your workload across multiple nearby data centers with a bit (or a lot) more tooling and services to help than if you were doing it in your own data centers.