Choosing a virtualization mode (PV versus PVHVM)
The terms PV and PVHVM refer to the virtualization mode used by the hypervisor to run the virtual machine. In general, PVHVM offers better performance than PV, especially for disk and network IO, but is not well supported in Linux operating systems with a kernel version earlier than 2.6.36. The availability of PV and PVHVM images in the Rackspace Cloud is determined by the effectiveness of each virtualization mode for that particular operating system.
If both PV and PVHVM options are available for the OS that you choose to use, consider the following factors when choosing between them.
- Network and disk IO are faster with PVHVM images because QEMU emulation is bypassed.
- PVHVM requires a bit more memory overhead than PV. If your application is memory intensive, or if it is optimized for PV Memory Management Units, PV might be a better choice.
- If you are striving for performance optimization, test both options to determine which one performs better for your application.
- Work-optimized servers (Compute, I/O, and Memory) require PVHVM images. If you try to create a work-optimized server by using a non-PVHVM image, the following error message is displayed:
"Image cannot be built with provided flavor."
- Ext3 is used for all Linux PV Linux images.
- Ext4 is used for all Arch, Fedora, Gentoo, and Ubuntu PVHVM images.
- PV images boot via pygrub.
- PVHVM images boot via the boot loader in the master boot record of the operating system.
Automatic disk configuration can be used with PV images but fails with PVHVM images. The error message looks as follows:
ERROR: Requested image $UUID has automatic disk resize disabled. (HTTP 400)
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