Number of VMKDs that can be concurrently monitored with IOInsight is limited to 20 in the current version due to the following reasons:
- IOInsight is mainly aimed at characterizing I/O behaviours of a workload or a VM. Generally number of VMDKs in a VM with active I/Os are less than 20 and this limit can satisfy most cases of monitoring 1 or a few VMs at a time.
- Monitoring large number of VMDKs at a time will have higher resource requirement (CPU, memory & network). So, this may even affect the performance of VMs in the host which is being monitored including the ones being monitored
- IOInsight collects stats at vmdk granularity and some info about every single I/O on monitored vmdks are sent to IOInsight from the host. So, IOInsight is suitable for a focused analysis where we are interested in a particular workload (VM) or a set of VM's. But tracking every I/O in a really large environment has it's own practical limitations. To get a high level overview of such a large system, other options like vCenter or vROps performance monitoring might be a more suited. But with those monitoring tools, you may not get the amount of details that is available with IOInsight.
So, you will have to either limit / narrow down the focus areas while using IOInsight by monitoring less than 20 VMDKs or use other tools which give more broader overview (host / cluster level), but with less detailed metrics.