How do I monitor my customer's ESXi host?

You can monitor an ESXi host using SNMP – OID. 

Examples of useful OIDs:

• Disk Failures – Disk failure is represented by object namehrStorageAllocationFailures, where the OID is.1.3.6.1.2.1.25.2.3.1.7.1. The value you should receive will be either 0 or other number representing the number of errors that occurred. In order to receive a Critical Alert in your Atera dashboard, you should add.1.3.6.1.2.1.25.2.3.1.7.1 to the monitored OID; then set a condition of greater than 0; and of course Alert Severity should be set to Critical.
• Memory (Used)– The server’s used memory is represented by object namehrStorageUsed, where the OID is.1.3.6.1.2.1.25.2.3.1.6.11. To get an alert on your Atera dashboard, add .1.3.6.1.2.1.25.2.3.1.6.11 to the monitored OID with the condition of greater than 4441124, and Alert Severity as Warning.
• CPU - The CPU entry is represented by object namehrProcessorLoad. Add the following pattern to the 'Discover' window: .1.3.6.1.2.1.25.3.3.1.2; you should receive a list of Cores. In order to monitor Core #8 use.1.3.6.1.2.1.25.3.3.1.2.8 to the monitored OID, with condition Greater than 60, and Alert Severity set to Critical

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To read more on adding an SNMP device click here