Internally, we use nagios. Externally, we use Panopta. Both services can get pretty granular, which is nice. You can monitor pesky services that stop for some reason or get alerts when you're reaching high disk usage. They are both very nice. Panopta offers SMS alerts as well.
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We do also use nagios to monitor 700 hosts and have 4000 serivce checks.
Right now we are also look at Observium for server/trend monitoring. The biggest win is that Observium tries to discover all it can monitor. So you never forget to add a new port/entity.
Here at Steadfast we use Zenoss to monitor our clients systems. This allows us to monitor any number of services or resource levels specifically geared around the customers needs. It can monitor anything from basic server resources (virtual, cloud or physical) to applications like mysql, mssql, postgre, apache and much more all in one place.
We use NewRelic server monitoring. It lets you track server health and you get alerts on server and memory utilization, health issues, disk capacity, etc. Read more here: http://newrelic.com/server-monitoring
Lots of lots of tools.
pingfinity.io for simple uptime monitoring on specific sites
pingdom.com for more uptime monitoring
and some internal monitor scripts, that also keep track of performance, etc.
We use Pingdom for exterior monitoring and they are great! Observium for internal changes, BGP session updates, etc. We also use various internal tools to monitor our devices.
Observium is a nice tool that we used for a few years, it worked seamlessly, but was not so easy to setup unfortunately...similar to Nagios, Cacti and Zabbix. Server monitoring market is quite undeveloped still. Good tools cost quite a lot of money, while free ones are difficult to setup for a person who has a small VPS and just needs to know it's status.
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