How do the alerts and alarms work in VMware thin provisioning?
Prior to the release of vSphere 5 there weren’t any vSphere-level alerts to warn users that a thinly provisioned data store was filling up. Thankfully, this has changed.
In vSphere 5 and above, the vSphere vStorage APIs for Array Integration automatically raise an alarm if a thinly provisioned data store is filled to 75% of its total capacity. When the alarm is triggered, the Storage Distributed Resource Scheduler no longer considers the data store as a potential destination because it is approaching maximum capacity.
Of course, this raises the question of what happens if the data store runs out of space. If that happens, virtual machines (VMs) requiring additional storage space are paused. Other VMs keep running until they require additional storage.
An alarm consists of a trigger and an action. A trigger is the condition that results when an alarm is generated. You can also attach a time value to a trigger. For example, you probably wouldn’t want to generate a low-memory alarm if memory usage spiked for a few seconds. The built-in memory alarm is only triggered if memory usage remains at 75% or higher for more than an hour or if it remains at 90% for longer than a minute.
When an alarm is triggered, an action is performed in response to the alarm. For instance, you might configure VMware to send an email notification.
It's worth noting that while the VMware thin provisioning storage alarm is useful, it's not the only use of alarms within VMware. VCenter Server includes a number of predefined alarms that monitor things such as clustering, hosts, individual VMs and licensing. You can also define your own alarms or modify existing alarms through the vSphere client.
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