Considering the fact that Apple
had close to zero
experience in making server hardware
, they did a nice job
. Not that the nifty design
matters much in server room
s, but the Xserve has some nice features
such as the special multi-channel hot-plug
IDE controller and the extensive status information
on the front
And the DDR memory, not yet very common in server hardware, was a complete primer for Apple. The G3 and G4 processors have a very slow (low-bandwidth) FSB, which made DDR memory pretty much useless in the Xserve - the processor can not utilise the bandwidth that the memory provides - making the FSB the bottleneck of the system.
So, can the Xserve use its DDR bandwidth? Yes, certainly. Because it has two processors, each connected to the memory with its own FSB. And also because in servers you have a lot of DMA access, where data is tranferred to and from RAM directly, bypassing the CPU.
A third possible reason why DDR may have been useful to Apple is an internal one. It could be that Apple used the Xserve as its first DDR platform to gain experience with faster memory types. Usually it is not a good idea to start experimenting with new technology in a server product, but DDR is not so much a new technology, rather an evolution of an existing one. And if you have to do the research anyway, it might be a good idea to do it first for a product with a high profit margin, paving the way for later products.
It shoud come as no surprise that Apple is able to make a nice, capable system in a 1U housing. After all, Apple is specialised in small motherboards: iMac, the TFT iMac, eMac, iBook and PowerBook all have tiny motherboards, and have lots of connectivity options. So Apple has plenty experience in making small, cool-running systems, and in making them reliable. And, along with sheer performance, that's exectly what you need in the server market today.
Update: the first major update to the Xserve line breaks the 32 bit barrier by using the 64 bit PowerPC 970 a.k.a G5 processors at 2 GHz, and extends the maximum physical memory size to 8 GB (ECC capable) RAM. In addition, it sports increased connectivity with dual USB 2.0, dual FireWire 800 and dual gigabit ethernet.
The major difference though, is that the new Xserve is no longer limited by legacy bottlenecks. The FSB bottleneck is gone. And the usefullness of a dual CPU (SMP) configuration has also increased: the G5 is known as one of the most efficient processors for SMP.
Once again Apple has done some solid engineering. It was believed that the new Xserves would have larger dimensions due to cooling constraints - a 2 units or 3 units rackmount form factor was expected instead of just 1U, but Apple managed to keep them just as small as they were. Although the G5s used in the Xserve produce less heat than those in the first-generation Power Mac G5s, they still have a much higher power disspipation than the G4s in the previous Xserve. Apple dealt with this by including air ventilation ducts at the front of the device, sacrificing the room for one (of the previously four) hot pluggable drive module.