When comparing Data Dog vs Nagios, the Slant community recommends Nagios for most people. In the question“What are the best server performance monitoring tools?” Nagios is ranked 6th while Data Dog is ranked 11th. The most important reason people chose Nagios is:
Being one of the oldest tools of its kind in the market, Nagios is extremely popular. This is usually a good thing because there are a lot of resources on the internet about it such as guides and configuration recipes.
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Pros
Pro Limited free tier
5 VMs with 24h history and no alerts for free.
Pro Docker support
The Datadog Agent container runs with the other containers on a host. It collects metrics from neighboring containers and from the host. It can monitor 10 or 20 containers free per host license.
Pro Popular
Being one of the oldest tools of its kind in the market, Nagios is extremely popular. This is usually a good thing because there are a lot of resources on the internet about it such as guides and configuration recipes.
Pro Scalable
Nagios was designed with flexibility and scalability in mind.
Pro Many open source options available
Cons
Con $15/host/month is quite expensive
If you are running big fat systems which are probably bare metal, DataDog may be a very good fit. But if you are running small virtual machines you may discover that its somewhat expensive price can become a real problem.