VMware vSphere 4 and Citrix XenServer 5.6 Pros and Cons

Virtualization is indeed a significant solution to maximize the resources utilization, increase reliability and availability. Further more, virtualization simplifies portability and administration and faster deployment.

Since virualization has a big market, it is also highly competitive. Having big players behind the solution should give us advanced key benefits over one another. Here we will try to understand the present top two Server virtualization products and their advantages and disadvantages.

The two big playes we will look at are: VMware vSphere 4 (ESX/ESXi 4.0) and Citrix XenServer 5.6 (beta).

Microsoft Hyper-V is comparatively immature and not quite ready to compete with vSphere and XenServer for large production environments and as well still missing major functionalities like: high availability, fault tolerance, load management, integrated VM management and administrating and only preferable for Microsoft base environments. So, while it might be a worthy mention at some stage, for now, we will focus on vSphere and XenServer.

VMware vSphere 4 (ESX/ESXi 4.0) Pros:

1.  VMware ESX/ESXi 4.0 offer minimal system– compact and small disk footprint size of 70 MB, besides Hyper-V and XenServer core installation took respectively 2 GB and 1.8 GB.

2.  vSphere infrastructure provide Efficient Resource Virtualization that includes:

– RAM over-commitment i.e. increase memory utilization like a server with 8GB physical memory can be 16GB.
Transparent Page Sharing e.g. several virtual machine are running same Widows Server 2003, they may have many identical memory pages. Transparent page sharing consolidates those identical pages into a single memory location.
Memory ballooning– Shift memory dynamically from idle virtual machines to active ones.
Support Paravirtualization spatially for network and storage drivers for more efficient network and storage access by virtual machines. Though VMware primary architecture was based on binary translation but they realize the Paravirtualization efficiency which was used by the XenServer along with hardware-assisted virtualization and afterward Hyper-V followed XenServer model.
VMDirectPath I/O that Allow virtual machines to directly access physical network and storage I/O devices to enhancing CPU efficiency in handling workloads.
– vSphere introduced Distributed power management that reduce energy consumption in the data center by optimizing workload placement. It’s only available with vSphere Enterprise edition.

3.  vSphere Infrastructure Scalability support 255GB RAM for virtual machines and upto 1TB RAM for large scale server consolidation and DR (Disaster Recovery) projects and each VMware ESX/ESXi Support for up to 256 powered-on virtual machines.

4.  vSphere provide broadest vendor supports for market-leading hardware,  Storage and OSs. Actually due to the VMware architecture (the first layer of software interacting with the hardware, i.e. need VMware-specific drivers) they have to support the driver for new hardware devices. This feature is an advantageous for the System architecture and management, but in some extent, it gives extra software driver development and that’s why we seen so many software updates and patches. As for a large and sophisticated system it will be complicated.

5.  vSphere Enterprise Plus edition offered extensive network management such as Centralize virtual network management, Simplified setup and monitoring of Private VLANs,  Segmenting network traffic easily in shared environments, Network VMotion.

6.  VMware use VMFS file system which is high performance file system that enables advanced features such as VMotion (Live migrate running virtual machines from one server to another with no disruption or downtime) and Storage VMotion. vSphere support dynamic VMFS volume resizing and also provide Boot from SAN that Simplify backups and disaster recovery by eliminating the need to separately backup local attached server disks.

7.  vSphere storage system add and extend virtual disks non-disruptively  to a running virtual machine to increase available resources. vSphere storage management provides Customizable reports and topology maps.

8.  vSphere High Availability and Disaster Recovery provide vStorage APIs for Data Protection, Backup proxy server that remove load from VMware ESX/ESXi installations, File level full and incremental backup.

9.  VMware VMFS file system is so comprehensive, moreover Vmware introduced Storage VMotion to allow to access different types of storage (FC, iSCSI, NFS and even local storage) as Citrix XenServer have unique storage integration features name ‘Citrix  StorageLink’.

10.  VMware High Availability and Fault Tolerance Provide zero downtime, zero data loss continuous availability against physical server failures with VMware Fault  Tolerance. XenServer Fault  Tolerance handled by third-party products like best-of-breed  Marathon Technologies and Stratus.

11.  VMware vShield Zones and VMSafe provide integrated dynamic firewalling for all applications, Control network access to sensitive areas and Logging and reporting. Citrix XenServer may use powerful Citrix Access Gateway with Advanced Access Control add-on. Both cases will come with price.

12.  VMware vCenter Server provides a central point of control for virtualization management, which is scalable and extensible management server for administering infrastructure and application services, with deep visibility into every aspect of virtual infrastructure. vCenter Server supports Event based alarms, Performance graphs and one vCenter Server can manage upto 300 hosts and 3000 virtual machines. Additionally, with vCenter Server Linked Mode may manage upto 10,000 virtual machines from a single console.

vSphere ESX/ESXi 4.0 Cons:

1.  VMware require more Patches and updates.

2.  vSphere only offer file-level backup and recovery, no application-level awareness.

3.  VMware vCenter required third-party database to keep the information’s of storage and management of host system configurations where XenServer do not required third-party database.

4.  VMware Distributed Resource Scheduler (DRS) feature, based solely on CPU and memory utilization, whether XenServer take care of CPU, memory, disk I/O, and network I/O for workload balancing (WLB).

5.  Though vSphere offer a wide range of features, but comes with a high price tag.

Cirtix XenServer 5.6 Pros:

1.  XenServer is built on the open-source Xen hypervisor and uses a combination of paravirtualization and hardware-assisted virtualization, which aware the guest operating system that it is being run on virtualized hardware. This collaboration between the OS and the virtualization platform enables the development of a simpler, leaner hypervisor, as well as highly optimized performance.

2.  XenServer offer an unique storage integration feature named: Citrix StorageLink, by which the administrator directly leverage features of industry-leading arrays such as from HP, Dell EqualLogic, NetApp, EMC and others.

3.  XenServer backup and recovery follows agent-based approach, by this customer get application-level awareness, such agent solution are: Windows VSS, SQL Server, Oracle, Exchange and Active Directory.  

4.  XenServer use sophisticated workload balancing (WLB), which captures CPU, memory, disk I/O, and network I/O data and offer two optimization modes: optimize for performance and optimize for density. Besides VMware Distributed Resource Scheduler (DRS) only based on CPU and memory utilization.

5.  XenServer offer a unique feature of Provisioning services, which enable multiple VMs to share a single workload image, resulting in streamlined administration as well as significant storage savings.

6.  XenServer is monitored and administrated by the XenCenter which may install on another windows PC or server (windows dependency!!), by this no load on XenServer host. XenServer core management services configuration information is kept on Server control domain and is replicated across all servers that are managed together to maintain high availability. Thus this architecture do not required separate database for core management functions. Though vCenter Server runs on separate machine but requires a third-party database for storage and management of host system configurations. So for redundancy and availability of the core management services VMware recommends additional clustering software such as its vCenter Server Heartbeat add-on product.

7.  Most important is XenServer offer the best free virtualization suite over other, which includes Multi-core processor support, Live migration, physical server to virtual machine (P2V) conversions and virtual-to-virtual (V2V) conversions tools, centralized multi-server management and real-time performance monitoring, high performance for Windows and Linux.

Cirtix XenServer 5.6 Cons:

1.  Large footprint than VMware and Relies on Linux in Dom.

2.  Few significant areas like: Core architecture, Hardware device drivers, Storage, Backup & Recovery, Fault tolerance XenServer required/depended (some cases additional feature) on third-party solutions.

Closing Thoughts

Finally to choose any server virtualization solution mostly depends on the customer’s specific requirements (basic or advanced features) and existing development tools & infrastructure. So, make a list of what you think you need and use that as your primary guide. Though VMware ESXi, Citrix XenServer and Microsoft Hyper-V offer free (limited) solutions, the licensing, Hardware, Management applications, Services and features, supports comes with high price if you do need the full function versions. As far as the free versions go, in my opinion, Citrix XenServer is best offered virtualization solution, then Hyper-V and then VMware.      


VMware vSphere 4 –
1.  VMware vSphere Features  – http://www.vmware.com/products/vsphere/mid-size-and-enterprise-business/features.html#c137533
2.  VMware Cost-Per-Application Calculator – http://www.vmware.com/technology/whyvmware/calculator/
3.  Compare vSphere Editions – http://www.vmware.com/products/vsphere/buy/editions_comparison.html

Citrix XenServer 5.6 –
1.  XenServer Enterprise-class and white paper – http://www.citrix.com/English/ps2/products/subfeature.asp?contentID=2300455
2.  XenCenter – http://community.citrix.com/display/xs/XenCenter

Microsoft Hyper-V –
1.  Hyper-V: Technical  Resources  – http://www.microsoft.com/windowsserver2008/en/us/hyperv-technical-resources.aspx
2.  Cost Comparison Calculator – http://www.microsoft.com/virtualization/en/us/cost-compare-calculator.aspx
3.  Advantages and disadvantages of Hyper-V – http://searchsystemschannel.techtarget.com/generic/0,295582,sid99_gci1313807,00.html

Other Links-
1.  Comparing Hyper-V R2, vSphere and XenServer 5.5 pros and cons – http://searchsystemschannel.techtarget.com/tip/0,289483,sid99_gci1379509_mem1,00.html
2.  Benchmarks: ESX vs Hyper-V vs XenServer – http://virtualization.info/en/news/2009/03/benchmarks-esx-vs-hyper-v-vs-xenserver.html

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