Blade servers are mini rackmount servers, designed for deployment in server rooms (data centers, corporate IT departments). They promise higher densities per rackmount frame, and lower power consumption per server.
Brands include Hewlett Packard (HP ProLiant BladeSystem), IBM (BladeCenter), Dell PowerEdge (M-Series), Cisco, Liquid Computing, NEC, Fujitsu, Oracle (Sun Microsystems), Bull, Hitachi, SGI and Verari Systems.
What is a Blade Server?
The basic blade server technologies (CPU, RAM, network cards etc) are similar to tower and rackmount servers. Blade servers are slotted (like expansion cards) into a special blade chassis or enclosure. It is the chassis that is rackmounted. The blades depend on the chassis for power and other support. Unlike a conventional rackmount server, the blades cannot function outside the chassis.
The chassis provides:
- A passive electrical backplane for DC electrical power, PCI and other connections to/from the blades.
- Power supply units.
- Cooling fans.
- KVM (keyboard, video, mouse) connections through the backplane, to allow control of multiple blades from a single console.
- Additional network connectivity (Fibre Channel, iSCSI over gigabit Ethernet, InfiniBand, FCoE) through the backplane.
Various types of CPUs (Intel, AMD, XEON, Opteron etc) are available, depending on the blade model. In general, blades from different manufacturers cannot be mixed and matched. Blades from one manufacturer will usually work only in a chassis from the same manufacturer.
Aside from server blades, other blades can be slotted into the chassis:
- Storage blades with hot-swappable SATA, SAS or other hard disks. They function like conventional disk arrays and have built-in RAID 0, 1, 5, 6 and 10 controllers. Some can even be configured as a SAN.
- Tape blades with a tape drive for data backup.
- PCI or PCI-X blades (containing empty sockets) to allow standard expansion cards to be connected to the server blades.
- Spacers to fill up empty slots, for proper airflow through the chassis.
Manufacturers usually provide full-height and half-height blades. As suggested by the name, two half-height blades can fit into the space for one full-height blade. For example:
- A HP c-Class 6U chassis holds 8 half-height or 4 full-height blades. The blades are slotted in horizontally.
- A HP c-Class 10U chassis holds 16 half-height or 8 full-height blades. The blades are slotted in vertically.
A typical half-height server blade will have:
- One or two CPUs.
- One or two 100 megabit/sec Ethernet cards.
- One or two hard disk drive bays.
- Two or three "mezzanine" connectors to connect to high speed network cards (Fibre Channel, gigabit Ethernet) on the chassis, via the backplane.
- One or two USB ports.
- No optical drive.
- No power supply.
- No cooling fans.
- No keyboard socket.
- No monitor socket.
A full-height blade will have more CPUs (up to 4) and hard disk bays. Storage blades are usually full-height blades, with up to 6 hard disk bays. Manufacturers usually allow half-height and full-height blades to be mixed in the same chassis.
Blade Server Advantages
Looking at the HP blade chassis mentioned earlier, it can be seen that 16 servers can be stored in only 10U of rack space. This compares favorably with 16U required for 16 individual 1U servers.
By pooling cooling fans and power supplies in the chassis, power consumption is reduced. Blade manufacturers provide Blade Power Calculators, to estimate electrical energy savings from switching to blade servers. There's a Blade Power Calculator from IBM. There's another Blade Power Calculator from HP. Blade server ROI (Return on Investment) calculators are also available (this one is from Dell).
Blade Server Disadvantages
One big disadvantage of a blade solution is the high up-front cost. A blade chassis can cost a few thousand dollars. This makes blades uneconomical if only a few servers are required. And depending on the specifications, a single blade server (excluding the cost of the chassis), usually costs more than a 1U server.
The cost savings from blades come from lower electricity costs and lower floor space costs. This is why ROI calculators are provided by manufacturers. If blade servers were cheaper than 1U servers, buying blades would be a "no-brainer."
Expandability is limited. Expansion PCI blades are available but they take up valuable slots on the chassis. A server that needs many PCI cards (for example, multiple network cards for multiple virtual machines) might be more suited to a single large rackmount server.
Blade Servers Versus Rack Servers
Blade servers are designed to reduce electrical power consumption and floor space. This is the point that is emphasized in many manufacturer's promotional materials. They are a good solution for data centers facing power or space constraints, and have the budget for the higher up-front hardware costs. Less constrained IT departments can afford to stick with simpler, cheaper and more flexible rackmount servers.
The individual blade manufacturers (such as IBM, HP and Dell), have more information on their blade products. Gartner has a report comparing the different blade manufactures, titled Magic Quadrant for Blade Servers.
Yuen Kit Mun - Kit Mun is a self-confessed information junkie, reading an average of a book a week over the past two decades. His growing Internet ...
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