Some of you know that I run a small fleet of Apple XServes. There are some in two-post racks at the house, and some at our super-secret hosting center in four-post cabinets. We're going to talk about what we've found with rack mount hardware and the older and newer generation computers.
For those unfamiliar with Apple's rack-mount servers, they're great boxes: easy to manage, good instrumentation, (better now with LOM), have great build quality and very easy part interchange. The G4 and G5 XServe models were designed to be serviced in the racks. This was a neat design, but a real pain in the neck in many cases, as the rack's not a great place to fix things. Further, the cable management system, although good in concept, required the use of a few feet of extra cable. The case design was basically a large sheath of metal into which the XServe was inserted. Thus, the XServe itself pulled out of the sheath and when removed from the rack had no protection for its innards. This made carrying the machine from one place to another a bit of a pain and also made keeping spare rack-mount kits a pain (since you needed the entire "lid").
The G4 and G5 XServe models were pretty easy to rack (especially after the folks at Apple expanded the rack mount kits in the G4 rev B machines to include 2-post rack mount kits). For the 2-post racks, using Apple's kit works just fine and is basically simple. The only trick is making sure you use the alignment guide to make sure that the sheath is appropriately mounted.
When Apple brought out the Intel XServes, they had given up on the idea of in- rack servicing and went with a more conventional mounting system involving rails and tracks. The lid is now a part of the server unit, and the only piece that remains in the rack is the rails on which the XServe slides in and out (see picture for the slides built into the case).
This is a pretty common configuration that has been seen from all sorts of vendors. Most of them don't accomodate 2-post racks in any form with these kinds of mounts, which is usually cause to go acquire mounting kits that act as shelves (such as those from RackSolutions).
Of course, I couldn't leave well enough alone, I decided I wanted to try and get them more securely into the rack. To that end, I put in a 2-post-to-4-post adapter (that allows you to hook in devices that attach to the front and back vertical rails). Unfortunately, due to the tight clearances of the Apple gear and some assumptions from Apple on how the racks will be configured, I could get neither the square-hole or round-hole rack rails to fit inside of my adapter.
I then tried our cheap shelf vendor's shelf. Nice idea, but unfortunately the screws in the unit stick into the path of the XServe and again the tolerances put a stop to our mounting.
In the end, I'm going to go with our known-good expensive shelf. We've used it in 4 post racks and I'm quite sure it will work here. Plus, they have a 2-post kit that has worked like a champ on XServe RAID boxes.
By the way, we've had trouble with any rack system that doesn't have a good quarter inch space available on the sides, beginning immediately behind the vertical rails built into the rack. In the case of our hosting center, we've had problems there due to the strange S shaped vertical rails... I'm sure they're sturdy, but they cut into space that Apple expects to use for their rails.
And for those who are curious, the racks at home look like this: