There are several pros and cons when choosing between blade or rack servers related to power consumption, consolidated networking, manageability and cost.  These comparisons are readily available over the Internet, but what makes one preferable to the other in a XenDesktop deployment?  Let’s take a look at a few of the differences.
 

Higher Density XenServer Resource Pools

Rack servers offer more quad-socket models compared to blades and also offer greater memory capacity and the latest processors.  This translates into higher VM density per server which is important because the size of a XenServer resource pool is limited by the number of XenServer hosts and not by the number of VMs.  So by using quad-socket rack servers you can have a greater number of virtual desktop VMs per XenServer resource pool.  In a large XenDesktop environment this means fewer resource pools to manage.
Advantage: Rack Servers

Isolated OS Streaming Traffic

Streaming the OS from the PVS host to the virtual desktop VMs can take significant network bandwidth especially when the virtual desktops are booting.  This OS streaming traffic can be isolated per blade chassis.  Each VM server would use the PVS host located in the same blade chassis but could failover to alternate PVS hosts in another chassis if needed.  Setting up an isolated network with rack servers is possible but is much more complex.
Advantage: Blades

Lower Storage Costs

Each virtual desktop VM needs a cache to temporarily store the “writes” while streaming a PVS standard image.  Depending on your environment this could require 1GB to 5GB or more of storage per virtual desktop VM but could also be used for storing EdgeSight user performance data or any persistent data such as anti-virus definition files.  Fast RAID storage options with local hard disks are possible with rack servers which is important because local storage is much less expensive than SAN storage even when using thin provisioning on the SAN.  Having this write back cache located on the same server will usually be easier to configure rather than putting all the write back caches on the SAN.  One downside is that dynamic load balancing of the running virtual desktop VMs is not available since the storage for the virtual desktops are local to the VM server.
Advantage: Rack Servers

Summary 

In conclusion both choices offer benefits in a XenDesktop environment.  I personally prefer rack servers because of their ability to use several local hard disks.  Keep in mind that your choices for network switches and network storage could significantly swing your decision one way or the other.  If you have any questions please feel free to leave a comment or email me.