IBM has been a top performer in recent server tests, which is why, for this hands-on exploration of blades vs. rackable servers, we tested IBM's state of the art in both categories - the HS21 and HS21 XM blades and the x3550 and x3650 rackables - and focused on performance, power consumption and manageability.

Because blade offerings have at least some things in common, we believe these test results are applicable generally to blades from other vendors - Dell and HP, for example - though there may be vendor-specific considerations in assessing products from others.

We found that blade servers reduce but don't eliminate redundant hardware components requiring electrical power, a pivotal consideration in total cost of ownership (TCO). That said, while IBM's blade servers equalled the performance of its rackables, the blades were more power efficient and potentially much easier to service.

While rackables are less expensive than blades when buying only one or two servers, because you have to factor in the cost of the blade chassis, a fully configured blade chassis is the more economical hardware buy than purchasing the same number of rackable servers. On the other hand, adopting a blade configuration demands a vendor lock-in that rackables don't, because they can be added one at a time as needed. Also, blades fall short for applications that require large amounts of on-board storage.

Product details

We tested two types of blades from IBM, the HS21 and HS21 XM. Both slid into the IBM Blade centre H (BC-H) chassis comprising a 10GB Ethernet switch, a 1GB Ethernet switch and four power supplies. The XM had a 2.33GHz Intel quad-core CPU, and the HS21 had a 2.0GHz CPU. IBM sent the XMs with 16GB of double-data-rate, second-generation (DDR2) memory, and the HS21s with 8GB of DDR2 memory.

In their specifications, these blades are comparable to the two x3550 rack servers that IBM sent, except that there's more space available inside the x3550s. IBM also sent one x3650 2U server, which included three RAID drives in RAID 5 configuration, a RAID controller and a hot-spare drive.

Prices for the blades ranged from around £3,500 for an HS21 with 2.0GHz twin-quad-core Intel CPUs with 8GB of memory, to around £5,000 for a hefty HS21 XM with a twin set of Intel 2.33GHz quad-core CPUs, 16GB of memory and an onboard RAID 0/1 configuration. Additionally, the BC-H chassis as specified costs about £9,000.

The rackables cost £4,000 to £5,000 depending on CPU clock-speed enhancements, extra memory and RAID controllers with extra drives.

Blade servers' drawbacks still prevent their adoption for some applications, such as those requiring large amounts of on-board, indeterminate storage expansion.

Even though performance tests among servers with comparable CPU clock and memory configurations were virtually identical, faster CPUs probably will arrive first in rackable form. This may not always be true, but our experience suggests any vendor can adapt the 1U or 2U form factor rapidly, while blade vendors have to plan in advance to accommodate new motherboard component combinations.