When you sign up for Datadog, you are immediately asked to choose whether you want to have your data stored in US1, US3, or Europe. This is an odd UI decision because Datadog provides no other information about US1 and US3, for example, where they are located or how old the infrastructure is in each place.

Fortunately we can use DNS to get some answers to these questions.

Where Datadog's US1 is located

The only info that Datadog provides about each site is the hostname; US1 is available at "app.datadoghq.com" and US3 is available at "us3.datadoghq.com".

Asking for the IP address of the A record for each service gives us a hint where they are located:

$ dig A app.datadoghq.com
; <<>> DiG 9.10.6 <<>> A app.datadoghq.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16096
;; flags: qr rd ra; QUERY: 1, ANSWER: 9, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;app.datadoghq.com.		IN	A
;; ANSWER SECTION:
app.datadoghq.com.	11	IN	CNAME	alb-web-2019-shard1-1762251229.us-east-1.elb.amazonaws.com.
alb-web-2019-shard1-1762251229.us-east-1.elb.amazonaws.com. 14 IN A 52.22.112.220
alb-web-2019-shard1-1762251229.us-east-1.elb.amazonaws.com. 14 IN A 18.205.215.170
alb-web-2019-shard1-1762251229.us-east-1.elb.amazonaws.com. 14 IN A 34.192.68.67
alb-web-2019-shard1-1762251229.us-east-1.elb.amazonaws.com. 14 IN A 54.210.66.118

So it looks like US1 is located in Amazon's us-east-1, a set of five datacenters in northern Virginia.

Where Datadog's US3 is located

Asking for the IP address of the A record for US3:

$ dig A us3.datadoghq.com
; <<>> DiG 9.10.6 <<>> A us3.datadoghq.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47450
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;us3.datadoghq.com.		IN	A
;; ANSWER SECTION:
us3.datadoghq.com.	299	IN	A	20.69.148.133
us3.datadoghq.com.	299	IN	A	20.69.148.69
us3.datadoghq.com.	299	IN	A	20.51.76.5

Doing a reverse IP address lookup reveals that these IP's belong to Microsoft in Washington state, so Datadog is likely using Microsoft Azure for data stored in US3.

Which one to choose

The primary consideration is travel time. It takes a packet about 40ms to travel across the USA, so each roundtrip is about 80ms. If your other data is located on the West Coast, you should consider US3; if your data is located on the East Coast, consider US1.

US3 is newer and may have newer hardware and better configuration; on the flip side, it may not be as well tested because it's the second one.

What about US2 and US4?

US2 and US4 return SOA results for a DNS query. US5 returns a Google data center located in Missouri (maybe that one is next?)

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