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What is Paravirtualization?

During a recent VMware TTT, an old option (no longer supported) to enable support for paravirtualization within a vm was mentioned.  Not everyone had come across this, so I decided to write a blog on it.

Paravirtualization is a virtualization technique that presents a software interface to virtual machines that is similar but not identical to that of the underlying hardware.

The intent of the modified interface is to reduce the portion of the guest’s execution time spent performing operations that are substantially more difficult to run in a virtual environment compared to a non-virtualized environment.  The paravirtualization provides specially defined ‘hooks’ to allow the guest(s) and host to request and acknowledge these tasks, which would otherwise be executed in the virtual domain (where execution performance is worse).  A successful paravirtualized platform may allow the virtual machine monitor (VMM) to be simpler (by relocating execution of critical tasks from the virtual domain to the host domain), and/or reduce the overall performance degradation of machine-execution inside the virtual-guest.

VMI is a paravirtualization standard that enables improved performance for virtual machines capable of using it.  The feature was available only for those versions of the Linux guest operating system that supported VMI paravirtualization.

Enabling paravirtualization used one of the virtual machine’s six virtual PCI slots.  Enabling paravirtualization could therefore limit how and where the virtual machine could be migrated.

Subsequently, Intel have introduced their VT technology, and AMD have AMD-V, both are hardware virtualization options that are ultimately faster then paravirtualization, and do not require the same modifications in the guest os.  For this reason paravirtialization as a technology has effectively been replaced which is almost certainly why in vSphere 5.0 it is no longer supported. (support ended in vSphere 4.1).

To find out how to enable it in vSphere 4, check out the VMware vSphere 4 – ESX and vCenter Server.

If you have this feature enabled in your guest os and wish to upgrade to vSphere 5 which as mentioned no longer supports this, there is a good KB article Migrating VMI-enabled virtual machines to platforms that do not support VMI.

Categories: vSphere
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