Low-cost enterprise virtualisation from XenSource

As I write this, I’m on the train to attend a Microsoft event about creating and managing a virtual environment on the Microsoft platform (that’s something that I’m doing right now to support some of my business unit’s internal systems). I’m also on the Windows Server Virtualization TAP program (most of the information I get from that is under NDA – I’m saving it all up to blog when it becomes public!) and I have a good working knowledge of VMware’s product set, including some of the (non-technical) issues that a virtualisation project can face. With that in mind, I thought I’d take the time to attend one of XenSource‘s Unify Your Virtual World events yesterday to look at how this commercial spinoff from the open source Xen project fits into the picture.

From my point of view, the day didn’t start well: the location was a hotel next to London Heathrow airport with tiny parking spaces at an extortionate price (at least XenSource picked up the bill for that); there was poor signage to find the XenSource event; and stale pastries for breakfast; however I was pleased to see that, low key as the event was, the presenters were accessible (indeed John Glendinning, XenSource VP for Worldwide Sales, was actively floor-walking). And once the presentation got started things really picked up with practical demonstrations supplemented with PowerPoint slides (not OpenOffice Impress as I would expect from an open source advocate) only to set the scene and provide value, rather than the typical “death by PowerPoint” product pitch with only a few short demonstrations.

XenSource logoXenSource was founded in 2005 by the creators and leaders of the Xen hypervisor open source project and in that short time it has grown to the point where it is now a credible contender in the the x86 virtualisation space – so much so that they are currently in the process of being acquired by Citrix Systems. Rather than trying to dominate in the entire market, XenSource’s goal is clear – they provide a core virtualisation engine with partners providing the surrounding products for storage, backup, migration, etc., ensuring that there are multiple choices for enterprises that deploy the XenSource virtualisation products. The XenSource “engine” is a next generation hypervisor which delivers high performance computing through its use of paravirtualisation and hardware assist technologies. They also try to cast off the view of “it’s Linux so it must be difficult” with their 10 minutes to Xen model with no base operating system or RPMs to install, demonstrating the installation of a Xen server on bare metal hardware in around 10 minutes from a PXE boot (other deployment options are available).

From an architectural standpoint, the Xen hypervisor is very similar to Microsoft’s forthcoming Windows Server Virtualization model, providing an environment known as Domain 0. Memory and CPU access is facilitated by the hypervisor, providing direct access to hardware in most cases although for Windows VMs to make use of this the hardware must support Intel-VT or AMD-V (virtualisation hardware assistance). Storage and network access use a high performance memory bus to access the Domain 0 environment which itself makes use of standard Linux device drivers, ensuring broad hardware support.

One of the problems with running multiple virtual machines on a single physical server is the control of access to hardware. In a virtualisation environment that makes use of emulated drivers (e.g. VMware Server, Microsoft Virtual Server) the guest operating system is not aware that it is running in a virtual environment and any hardware calls are trapped by the virtual machine management layer which manages interaction with the hardware. The paravirtualised model used for Linux VMs allows the guest operating system to become aware that it is virtualised (known as enlightenment) and therefore to make a hypercall (i.e. a call to the hypervisor) that can interact directly with hardware. For non-paravirtualised operating systems that use the high performance memory bus (e.g. current versions of Windows), full virtualisation is invoked whereby the virtual machine believes it owns the hardware but in reality the hardware call is trapped by the virtualisation assist technology in the processor and passed to the hypervisor for action. For this reason, Intel VT or AMD-V capabilities are essential for Windows virtualisation with Xen.

XenSource view the VMware ESX Server model of hypervisor-based virtualisation as “first generation” – effectively using a mini-operating system kernel that includes custom device drivers and requires binary patching at runtime with a resulting performance overhead. In contrast, the “second generation” hypervisor model allows for co-operation between guests and the hypervisor, providing improved resource management and input/output performance. Furthermore, because the device drivers are outside the hypervisor, it has a small footprint (and consequentially small attack surface from a security standpoint) whilst supporting a broad range of hardware and providing significant performance gains.

XenSource claim that paravirtualised Linux on Xen has only a 0.5-2% latency (i.e. near-native performance) and even fully virtualised Windows on Xen has only a 2-6% latency (which is comparible with competing virtualisation products).

There are three XenSource products:

  • XenExpress – a production-ready, entry level system for a standalone server (free of charge).
  • XenServer – a mid-range multi-server virtualisation platform
  • XenEnterprise – high capacity dynamic virtualisation for the enterprise.

Because the three products share the same codebase (unlike Microsoft Virtual PC/Virtual Server or VMware Workstation/Server/ESX Server), upgrade is as simple as supplying a license key to unlock new functionality. For XenServer and XenEnterprise, there are both perpetual and annual licensing options (licensed per pair of physical CPU sockets) at a significantly reduced cost when compared with VMware Virtual Infrastructure 3 (VI3).

The version 4 XenSource products were released in August 2007 with an update planned for the last quarter of 2007. New features in version 4 include:

  • XenMotion (XenEnterprise only) for seamless movement of virtual machines between hosts without any noticeable downtime (cf. VMware VMotion).
  • XenResourcePools (XenEnterprise only) to join virtual servers and manage virtualised resources as a logical group, supporting automatic VM placement and XenMotion with shared storage (volume-based iSCSI and file-based NFS, using the .vhd disk format), authentication, authorisation and resource configuration (similar to the model in VMware Virtual Center).
  • Xen64, a true 64-bit hypervisor providing scalability and support for enterprise applications in either a 32- or 64-bit environment with quality of service controls on resources, dynamic guest configuration and supporting up to:
    • 128GB RAM (32GB per guest, hotplug addition for supported Linux operating systems).
    • 1-32 pCPUs (1-8 vCPUs per guest).
    • 1-8 NICs (1-7 NICs per guest – hotplug addition and removal).
    • 1-128 storage repositories (16TB per repository with hotpluggable disks).
  • XenCenter, which provides a graphical virtualisation management interface, with guided wizards and guest templates for host and resource pool configuration on multiple servers, storage and networking configuration and management, VM lifecycle management and import/export (cf. VMware Virtual Center). Whilst CLI commands are also available XenCenter is a Microsoft.NET application for Windows operating systems which makes use of the latest Windows user interface standards. Because XenCenter makes use of a distributed configuration database there is no dependency on a single SQL Server and management can fail over between virtual host servers.
  • XenAPI, a secure and remoteable programming interface for third-party and customer integration with existing products and processes including the xe commands for system control.

One example of the XenSource approach to providing additional functionality through partnerships is the agreement with Symantec whereby Symantec (formerly Veritas) Storage Foundation will be embedded into XenEnterprise (providing dynamic fibre-channel multipathing for redundancy, load balancing, resilience and speed); a new product called XenEnterprise High Availability will be developed for virtual machine failover; and Veritas NetBackup will be offered for data protection and backup of critical applications running on XenEnterprise virtual machines (via the NetBackup Agent, also supporting snapshots when used with Symantec Storage Foundation). Rather than re-certify systems for virtualisation, XenSource will accept Symantec’s certified plugins for common OEM architectures and, because Symantec Storage Foundation is already widely deployed, existing investments can be maintained.

In terms of demonstration, I was impressed by what I saw. XenSource demonstrated a bare metal installation in around 10 minutes and were able to show all the standard virtualisation demonstrations (e.g. running a ping, copying files, or watching a video whilst performing a live migration with no noticeable break in service). The XenCenter console can be switched between VNC and RDP communications and Xen makes use of is own .xva Xen virtual appliance format with Microsoft .vhd virtual hard disks. Conversion from VMware .vmdk files is possible using the supplied migration tools (there are Linux P2V tools included with the XenSource products but for Windows migrations it’s necessary to use products from partners such as PlateSpin and LeoStream) and templated installations can also be performed with simple conversion between running VMs and templates. When cloning virtual machines, there are options for “fat clones” whereby the whole disk is copied or thin provisioning using the same image and a differencing drive. Virtual machines can use emulated drivers or XenSource Tools can be installed for greater control from the console. Storage can be local, NFS or iSCSI based with fibre channel storage and logical volume management expected in the next release.

It’s clear that XenSource see VMware as their main competitor in the enterprise space and it looks to me as if they have a good product which provides most of the functionality in VMware VI3 Enterprise Edition (all of the functionality in VMware VI3 Standard Edition) at a significantly lower price point. The Citrix aquisition will provide the brand ownership that many sceptics will want to see before they buy an open source product, the partnership model should yield results in terms of flexibility in operations and it’s clear that the development pace is rapid. With XenSource going from strength to strength and Microsoft Windows Server Virtualization due to arrive around the middle of next year, VMware need to come up with something good if they want to retain their dominance of the x86 virtualisation market.

3 thoughts on “Low-cost enterprise virtualisation from XenSource


  1. I just got an e-mail from Citrix to say that their acquisition of XenSource is now complete and along with that are a few product name changes:

    Citrix XenServer becomes the product family name.
    XenExpress becomes Citrix XenServer Express Edition.
    XenServer becomes Citrix XenServer Standard Edition.
    XenEnterprise becomes Citrix XenServer Enterprise Edition.

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