One of the political-like eternal debates is the debate around vSphere Hosts Form Factor, which to choose: Blade Servers or Rack-mounted Servers. Both technologies are mature and support high computing power and Hardware Vendors offer both of them now equally. In addition, VMware vSphere supports using both of them and puts no limitation on the form factor of the hosts. They’re dominating now over Tower form factor which began to disappear because of its large foot print and high power usage. Confusing to choose between them, right?
Long story short, both options have their own Pros and Cons that should be aligned with your (customer) case and your (customer’s) requirements and constraints. In the following table, a summary of the main differences between both form factors mentioning the aspect. It’s not vendor-related, some other differences might exist according to the vendor(s) you’re comparing.
|Blade Servers||Rack-mounted Severs|
|Cost||Doesn’t increase linearly. It increases little for single blade, then with huge amount (nearly 30k) for a new Chassis and hence, many blades should be purchased with the first blade chassis to justify its initial price. However, cost reduction is massive for Switches, FC Fabrics, cablings and power equipment.||Increases linearly with low initial cost, as you can purchase just even single rack server. Additional cost is needed for Switches, FC Fabrics, cablings and power equipment per each rack server.|
|Availability||Some Single Points of Failure exists, as failure of a Chassis or a midplane, although extremely rare, will lead to failure of many servers.||No single point of failure here, but for an entire rack failure (extremely rare). Failure of a single Rack-mounted server will lead to failure of single host.|
|Manageability||Blade Servers requires co-operation between different teams (Network, Storage, etc.) as well as deep knowledge of management of this blade vendor.||Rack servers are known entities and their manageability is relatively easy compared to Blade Chassis and Blade Servers. Management separation between teams still exists as usual.|
|Networking Configuration & Cabling||-Require less cabling.
– Harder to configure its networking and harder to manage.
|– Require more cabling
– Easier to configure their networking and to manage.
|Performance||Blade servers have less space inside and hence lower RAM or less IOps, but a fully-populated blade chassis in some U’s may provide Computing Power higher than a group of rack-mounted servers occupying a single rack.||Rack-mounted servers have wide space inside and hence many RAM, PCI Cards and Disks slots.|
|Security||Blade Servers enforce the use of VLANs for logical separation between networks. Sometimes, Blade Servers can’t be used when physical separation is required.||Rack servers may be required for physical separations of workloads or network traffic.|
|Upgrade & Scalability||Blade Serves are really limited in their upgrades and scalability. You’re stuck in a couple of blade generations till replacing the entire blade chassis. although it’s not needed usually, adding modules in a blade server is very limited.||Rack servers can’t be matched in their upgradability and scalability. Any HW piece (CPU, RAM, IO Card, etc.) can be upgraded individually.|
|Density||High number of servers in a small foot print, usually some U’s.||Low density.|
|Environmental Issues||Uses low power consumption per blade and per blade chassis, however a full blade chassis produces a concentrated heat in a small place and hence, requires concentrated cooling.||High power usage per rack server. Entire rack can hold limited number of servers and hence, a full rack produces less amount of heat than a blade chassis and requires less cooling.|
Both options are strong valid options. As nothing is perfect, both Rack-mounted and Blade servers have their own Pros and Cons.
Define your (customer’s) requirements and constraints then according to the case define which option will suit you (your customer).
Finally, I’ll conclude with a quote from vSphere Design Sybex 2nd Edition by Forbes Guthrie, Scott Lowe and Kendrick Coleman:
“Both blades and rack-mounted servers are practical solutions in a vSphere design. Blades excel in their ability to pack compute power into datacenter space, their cable minimization, and their great management tools. But rack servers are infinitely more expandable and scalable; and they don’t rely on the chassis, which makes them ultimately more flexible.
Rack servers still dominate, holding more than 85% of the worldwide server market. Blades can be useful in certain situations, particularly in large datacenters or if you’re rolling out a brand-new deployment. Blades are inherently well suited to scale-out architectures, whereas rack servers can happily fi t either model. Blades compromise some elements. But if you can accept their limitations and still find them a valuable proposition, they can provide an efficient and effective solution.”