Virtualisation: Xen is looking to catch up by releasing version 4
Version 4 of the free Xen hypervisor has been available to download since the 7th of April. In many respects, the new version has caught up with current developments, especially in terms of the Linux kernel. In addition, Xen 4 sports various new features that make it stand out against alternatives such as KVM, which comes with the Linux kernel; among these is that the PCI device pass-through in virtual machines now also works with graphics cards.
The frequently criticised point that Xen is based on an obsolete Linux kernel (official implementations were previously based on 2.6.18) has been straightened out by the developers: Xen 4 uses the Linux kernel's official virtualisation interface (pv_ops) and uses as Linux 2.6.31 as its default kernel version. However, the developers will also continue to provide patch sets for 2.6.18 and 2.6.32 (the Linux kernel developers are currently working on 2.6.34).
Xen 4 is capable of managing 128 virtual and 128 physical CPUs, as well as one terabyte of main memory, per system. On systems which offer I/O virtualisation, PCI cards as well as graphics cards can be passed through to a virtual machine. The latter is only possible with fully virtualised systems (HVMs) such as Windows guest systems. In addition, the system's BIOS and motherboard must co-operate if a virtual machine is to succeed in directly accessing a graphics card. The Xen developers have listed some selected components that are suitable for this purpose.
The integration of blktap2 allows Xen to handle the VHD format, which is Microsoft's image format for disks including those of virtual machines. blktap2 also know about snapshots and sparse images. By incorporating netchannel2, the developers have extended Xen's networking functions to accommodate hardware that is optimised for virtualisation. Xen now also works with a more modern Grub (version 2) for guest systems. A new C interface (libxenlight) is to improve management software connectivity.
Windows guests will from now on also be able to use Microsoft's certified drivers for optimising disk and network access in the free Xen. This considerably speeds up virtualised Windows systems under Xen. The drivers can be found on the Xen cloud platform. Xen 4 can share identical memory pages across several HVM guest systems. The developers have incorporated TMEM to further optimise the utilisation of the main memory managed by the hypervisor; it allows guest systems to place unused memory in a VM-independent pool or claim it from there.
PV-USB is now also included: These additions allow both para-virtualised guest systems and unmodified guests (HVMs) running under Xen to access USB devices that are connected to the actual host. This feature was previously only available on an experimental basis and limited to USB 1.1 in HVMs. Now, Xen 4 wants to offer USB 2 performance. Other new functions (originally part of the Remus project) synchronise the state of running virtual machines via a network.
Further details on the GPL licensed Xen 4.0 are available in the release notes.