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Please note: to ensure the execution of the vSphere Distributed Resource Scheduler (DRS) and to offer High Availability (HA) for your Virtual Machine, it may be necessary to automatically unmount ISO images from your VMs.

If an ISO image is still needed by VM, the ISO can be (re)attached using either of the following methods:

Using the ITS Private Cloud API. How to mount a custom ISO to a Virtual Machine.
Using the WebUI to (re-)attach the desired ISO file to the VM's CD/DVD device. Load an ISO image in a Virtual Machine

What is VMware DRS?

VMware DRS dynamically balances computing capacity across a collection of hardware resources aggregated into logical resource pools, continuously monitoring utilization across resource pools and intelligently allocating available resources among the virtual machines based on pre-defined rules that reflect business needs and changing priorities. When a virtualmachine experiences an increased load, VMware DRS automatically allocates additional resources by redistributing virtual machines among the physical servers in the resource pool.

VMware DRS allows IT organizations to:

Prioritize resources to the highest value applications in order to align resources with business goals
Optimize hardware utilization automatically and continuously to respond to changing conditions
Provide dedicated resources to business units while still profiting from higher hardware utilization through resource pooling
Conduct zero-downtime server maintenance

References

"VMware Distributed Resource Scheduler (DRS)." VMware Inc. 2009. Web. 4 Sept. 2015. <http://www.vmware.com/files/pdf/VMware-Distributed-Resource-Scheduler-DRS-DS-EN.pdf>.