Blade servers vs. rack servers

Blade limitations

The amount of onboard blade-server storage media is limited, a function of the small size of a blade and its heat dispersion needs. The blades we tested could have RAID configurations on them, but only a RAID 1 mirroring or RAID 0 non-redundant disk striping. This means that capacities are bound by the size of the drives vendors ship with the blades, in this case, a limit of 288GB (maximum; we tested two 73GB drives in the blades) from IBM. Some will argue that 288GB is a lot, but it's a ceiling amount unless iSCSI, Fibre Channel or other SAN connection methods are onboard and well configured.

By contrast, the 2U x3650 can house a RAID 5 configuration internally by using four drives - three for RAID and one as a hot-spare. These drives, like those in blade-server housings, can be changed to accommodate larger storage needs, but it's unlikely that administrators will change drives once a unit is in service for a production application.

With more native space available, rack-mounted servers have a higher denominator of native storage capability. Some vendors are putting this same storage capacity in 1U form-factor rackable servers as well. There's a tendency not to change components in a working system even if better components will achieve a longer application or service profile life.

The result is that blades need to connect to an external SAN if it's known or perceived that future storage-growth needs will exceed the amount of space available on the blade. For organisations that have SANs, blade-server attachments are well understood, and a variety of Fibre Channel SAN gear is available. If there's no SAN nearby, however, iSCSI can be used, subject to the bandwidth consumed over Ethernet connections by the virtualised iSCSI-based traffic.

It's also possible to put additional I/O cards into blades, as well as into rackables. The IBM line uses PCI and PCI Express (PCIe) cards in its rackables. Options to increase to two cards makes rackables more flexible in terms of I/O card expansion. The memory options in blades and rackables are similar.

Another potential disadvantage to blades is commitment. If you choose a blade-server vendor, your organisation becomes captive to that vendor's service policies, component availability and service organisation. This isn't necessarily the case for 1U deployments, because the 1U space can be occupied by any vendor, but a blade space will be occupied by a server from the chassis' manufacturer. Relationships become tighter if a blade environment is chosen.

Bottom lines

Blade density represents a huge pool of computing power per cubic inch. Typically, four 9U IBM blade chassis will fit in a 42U rack with space to spare, which means almost 20,000 watts of power to cool. Forty x3550 rackable servers in the same rack use at most 75 percent of that wattage. There are 56 blades in this configuration, however, vs. 40 rackables inside a single rack, maximised in this way.

Performance was essentially the same between rackables and blades. Blades are easier to manage and service. We believe there's a vendor-captivity element to purchasing or deploying blade servers, as well as a decided limitation on local storage. However, in an organisation with very good datacentre infrastructure, if you can live with the vendors as in-laws, you'll love the blades.

Henderson is principal researcher, and Dvorak, a researcher at ExtremeLabs in Indianapolis, USA.