XenServer, Hyper-V, and VMware Hypervisors Go to the Mall

Shortly after the Christmas rush, someone asked me to explain the difference between Citrix XenServer, Microsoft Hyper-V, and VMware. After giving it a little thought, I came up with the following explanation. It was a bit unique, and thought that I would share it with you all.

To begin with, think of yourself around the holidays with a list of presents to buy at the Mall. If you have ten purchases from ten different stores to make this afternoon, it may take you a couple of hours to get through the entire list. However, if you had to bring five kids with you to the mall for the day, it would take you significantly longer to get through the same shopping list. The overhead of managing the childrens’ needs during the trip adds a good deal of overhead to your workload that impacts the amount of time you dedicate to present acquisition.

This scenario plays itself out when comparing Xen and Hyper-V to VMware’s ESX. VMware’s ESX is a ‘bare metal’ installation. This means that the hypervisor is installed directly onto the server. At only 32 MB in size, ESX is optimized to manage the Virtual Machine I/O and hardware translation with very little overhead. It also allows ESX to over allocate resources such as memory leveraging direct control over the physical resources.

With XenServer and Hyper-V, both installations use a base or core OS. This installed Operating System manages the hardware similar to what happens in any other server installation. The hypervisor is then installed and is run in the core OS. It is this hypervisor that manages the interaction between the hosted VMs and the hardware. The significant difference is that the core OS manages the hypervisor rather than having the hypervisor installed directly onto the hardware itself. With the OS management as a filter/management interface between the hypervisor and hardware, it offers a layer of complexity that is absent in the VMware ESX offering.

Similar to bringing along several kids with you to the mall, a hosted hypervisor will always have additional overhead associated with the core OS that you won’t have with VMware’s ESX. Additionally, because ESX controls the hardware directly, there is significant savings that can be realized in resource over allocation that can’t be found in hosted hypervisors. Hosted hypervisors can only allocate a subset of their physical resources to VMs, where ESX can optimize and prioritize the resources to get the most out of the physical assets.

So next time you are wondering about the difference between hypervisors, think of your last trip to the mall and smile.

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Citrix Free XenServer a Challenge for VMware

The recent announcement by Citrix indicating that they are releasing their flagship virtualization engine, XenServer, for free is a bold step towards increasing their market share in the SMB space. After an anemic 2008, where they posted only 4% growth and downsizing 10 percent of their staff, this move puts them in the forefront of the news.

Citrix XenServer offers virtualization based on the open source Xen engine. This is the same release that was previously sold as the enterprise product. Included in the package is their multi-node management interface, live motion of VMs from one node to another, and integrated storage management to support host based logical volume management. These features are more versatile and diverse than VMware’s free offering, ESXi, giving them an edge in the free hypervisor marketplace. While comparison to ESXi grants Citrix XenServer the edge with features such as Live Motion that aren’t offered by VMware without VCenter and licensing.

This puts VMware on notice that they will have significant competition in the SMB space. XenServer has more features than the free ESXi offering, but not as mature of feature rich as VMware’s VI3 Virtual Infrastructure. I don’t see enterprise customers looking to Citrix, but at the price point, it may open a few doors in the smaller marketplace.

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What is the basic difference between VMware and Hyper-V

(This answer is adapted from a Techscrawl.com blog entry “VMWare ESX / Microsoft Hyper-V Comparison” Posted on 14 Aug 2008 by Clay)

Microsoft Hyper-V and VMware ESX are hypervisor based solutions. They install directly on the hardware and require no lower level OS beneath them, however their architecture is quite different.

The hypervisor is a critical component of and foundation of virtual infrastructures. Fundamental characteristics of a hypervisor are:

  • Have a purpose-built, thin OS independent architecture for enhanced reliability and robustness
  • Make optimal use of available hardware resources
  • Deliver performance acceleration features that support mission critical applications
  • Enable advanced capabilities not previously possible on physical systems

ESX installs a hypervisor on the hardware. It acts as the intermediary between the hardware and any virtual machines running on the server. Hardware device drivers are included in the hypervisor. This is called a direct driver model.

Hyper-V also installs on bare metal. But all management functions and access to hardware is controlled via a “root partition” that runs the Windows Server (or Server Core) 2008 OS. This root partition is actually a special virtual machine, through which hardware I/O requests from child partitions travel via the VMBus architecture. This is called an indirect driver model. So basically before you enable the Hyper-V role, your server OS is of the typical architecture, after enabling the role, Hyper-V installs itself on top of the hardware, and places your original OS into this special virtual machine, the root partition.

A comparison of certain key features between platforms:

  • ESX supports both 32 & 64-bit hosts, Hyper-V requires a 64-bit host that supports hardware-assisted virtualization. All platforms support 32 or 64-bit guests.
  • Maximum Logical Host CPU’s: ESX = 32, Hyper-V = 16 (can do more, but not supported)
  • Maximum Supported Host Memory: ESX = 256 GB, Hyper-V = 2 TB (2008 Enterprise Ed.)
  • Maximum Memory per Guest OS (VM): ESX & Hyper-V = 64 GB
  • Maximum Supported Running VM’s: ESX = 128, Hyper-V = limited only by available resources
  • RAM Over-Commitment: Supported in ESX, not supported in Hyper-V. (This allows RAM allocated to VM’s to exceed actual available RAM in host).
  • NIC Teaming: Native support in ESX. Hyper-V only supports via 3rd party drivers.
  • Maximum # Virtual Switches: ESX = 248, Hyper-V = unlimited
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What is VMotion?

VMware VMotion enables the live migration of running virtual machines from one physical server to another with zero downtime, continuous service availability, and complete transaction integrity. It is transparent to users.

VMotion lets you:

  • Automatically optimize and allocate entire pools of resources for maximum hardware utilization and availability.
  • Perform hardware maintenance without any scheduled downtime.
  • Proactively migrate virtual machines away from failing or underperforming servers.

So how Does VMotion work?

First, the entire state of a virtual machine is encapsulated by a set of files stored on shared storage. VMware’s clustered Virtual Machine FileSystem (VMFS) allows multiple installations of ESX Server to access the same virtual machine files concurrently.

Second, the active memory and precise execution state of the virtual machine is rapidly transferred over a high speed network. This allows the virtual machine to instantaneously switch from running on the source ESX Server to the destination ESX Server. VMotion keeps the transfer period imperceptible to users by keeping track of on-going memory transactions in a bitmap. Once the entire memory and system state has been copied over to the target ESX Server, VMotion suspends the source virtual machine, copies the bitmap to the target ESX Server, and resumes the virtual machine on the target ESX Server. This entire process takes less than two seconds on a Gigabit Ethernet network.

Third, the networks used by the virtual machine are also virtualized by the underlying ESX Server. This ensures that even after the migration, the virtual machine network identity and network connections are preserved. VMotion manages the virtual MAC address as part of the process. Once the destination machine is activated, VMotion pings the network router to ensure that it is aware of the new physical location of the virtual MAC address. Since the migration of a virtual machine with VMotion preserves the precise execution state, the network identity, and the active network connections, the result is zero downtime and no disruption to users.

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Can I run Linux and Windows on the same ESX server?

With VMware ESX, you can virtualize any environment, from the corporate data center to the branch office, with a compatibility list that includes more than 200 server and storage systems, and a broad range of supported guest operating systems, including Windows, Linux, Netware, and Solaris. And due to the VMware’s inherent Virtual Machine encapsulation each OS is isolated from others on the physical server. This means that yes you can run different OS flavors on a single physical server.

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How secure is VMware and Virtual Machines?

VMware is quite secure. One high level national security organization spent six months trying to crack VMware virtual machines. They ended up implementing it.

VMware Infrastructure addresses security in a number of ways. Some of these include:

  • Compatibility with SAN security practices. VMware Infrastructure enforces security policies with LUN zoning and LUN masking.
  • Implementation of secure networking features. VLAN tagging enhances network security by tagging and filtering network traffic on VLANs, and Layer network security policies enforce security for virtual machines at the Ethernet layer in a way that is not available with physical servers.
  • Integration with Microsoft® Active Directory. VMware Infrastructure allows you to base access controls on existing Microsoft Active Directory authentication mechanisms.
  • Custom roles and permissions. VMware Infrastructure enhances security and flexibility with user-defined roles. You can restrict access to the entire inventory of virtual machines, resource pools and servers by assigning users to these custom roles.
  • Resource pool access control and delegation. VMware Infrastructure secures resource allocation at different levels in the company. For example, when a top-level administrator makes a resource pool available to a department-level user, all virtual machine creation and management can be performed by the department administrator within the boundaries assigned to the resource pool.
  • Audit trails. VMware Infrastructure maintains a record of configuration changes and the administrator who initiated each one. You can export reports for event tracking.
  • Session management. VMware Infrastructure lets you discover—and if necessary—terminate VirtualCenter user sessions.
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Tim Antonowicz
Tim Antonowicz
Senior Sales Engineer

Tim has over 15 years of experience in Enterprise storage and backup, Virtualization, Disaster Recovery, Messaging, and cross-platform integration.

Tim is nationally recognized in the fields of Virtualization, Disaster Recovery, and Security, frequently speaking at national conferences such as VMWorld, HP Technology Forum, EduCause, Gartner MES and FinSEC.

He is also a charter member of VMWare's SMB Customer Advisory Panel and sits on the New England VMWare User Group's Steering Committee.

Tim has a Bachelor of Science degree in Biology from the Worcester Polytechnic Institute and holds various industry certifications including the VMware Certified Professional, Microsoft Exchange 2003, and has been a Microsoft Certified Systems Engineer since 1998.

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