I/O Virtualization

I/O is a big issue for any form of virtualization. If your virtualized server lacks CPU power, you can just add more CPUs or cores (i.e. replace dual-core CPUs with quad-cores). However, the memory bandwidth, the chipset, and storage HBA are in most cases shared by all virtual machines and a lot harder to "upgrade". Moreover, contrary to the CPU, the rest of the hardware in most virtualization software is emulated. This means that each access to the driver of a virtual hardware component must be translated to the real driver.

A real 3.46 GHz Intel Xeon processors runs on an emulated BX-chipset: we are running inside a VM.

If you inspect the hardware of a virtual machine in ESX for example, you can see that modern CPUs have to work together with the good but nine years old BX chipset, and that your HBA is always an old bus logic or LSI card. This also means that the newest tricks that your hardware uses to improve performance cannot be used.

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