This work shows the optimizations we have been investigating and implementing at the KVM virtualization layer in our research institute based on more than a year of experience in running thousands of VMs in a production environment used by several international collaborations. These optimizations increase the adaptability of virtualization solutions to demanding applications like those run in our institute (High-Energy Physics). In particular we will show what is the best solution to share

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... ge space from an hypervisor to the virtual machines running on top of it, trying to avoid problems that modern storage cluster have. We will also show how different the performance can be comparing real and virtual machines, measuring access to computing, storage and network resources, trying to provide hints on server configuration where possible. This work has been driven by the project called Worker Nodes on Demand Service (WNoDeS), developed by INFN, a framework designed to offer local, grid or cloud-based access to computing and storage resources, preserving maximum compatibility with existing computing center policies and work-flows.