User:Pepoluan/Sandbox/XenServer

Citrix XenServer is a complete, managed server virtualization platform built on the powerful (and Open Source) Xen hypervisor. Similar to VMware ESX/ESXi and Microsoft Hyper-V, XenServer is a baremetal (Type 1) hypervisor

Since XenServer 'centers on efficiency and performance first', you need to understand some subtleties in deploying Gentoo VMs (Virtual Machines) on XenServer

PV vs HVM
XenServer actually provides two types of virtualization:


 * Paravirtualization (PV) -- 'guest' VMs use specially-modified operating systems that relies on special 'hypercall' ABIs instead of the 'normal' ABIs. This allows near-native performance.


 * Hardware(-assisted) Virtual Machine (HVM) -- guest VMs use non-modified operating systems; XenServer will rely on the processor's 'virtualization extensions' (Intel VT-x or AMD-V) to intercept hardware calls, which will be handled (emulated) by QEMU. This evidently require a processor that implements said extension. Although performance is still much higher than straight binary translation/emulation as used by VMware, the quite significant emulation by QEMU will ultimately throttle the guest VMs' performance.

Linux kernels since 2.6.37 have built-in (i.e., in-kernel) support for PV mode, allowing Linux distros (with a recent enough kernel) to run at near-native performance.

PV-on-HVM
In order to close the gap between PV-mode performance and HVM-mode performance, the Open Source community has released so-called "PV-on-HVM" drivers. These drivers provide increased I/O performance because, instead of handling I/O via QEMU emulation, these drivers 'shunt' I/O requests as direct hypercalls to the XenServer hypervisor.

These drivers are required by proprietary operating systems such as Microsoft Windows, but not really required by Linux distros since the latter can run in 'pure' PV-mode

On Xen, there are two XenServer guest VMs can run in PV-mode:


 * By providing a PV-mode-supporting kernel within the Dom0 filesystem, or
 * By booting a PV-mode-supporting kernel inside the guest VM using  or

On XenServer, only the second method is available.

Running Gentoo on XenServer
Because of the sheer size/amount of tuning required to eke the utmost performance of Gentoo as a guest VM, the HOWTO has been split into several standalone articles:


 * XenServer/Installing Gentoo PV DomU
 * XenServer/Tuning DomU kernel