A drawback of short servers
Since Oracle killed off the inexpensive SunFire line, our new generation of 1U servers is from Dell and we've started migrating to them. One of the two models is the Dell R210. It has two features of particular note. The first is that it doesn't have easily accessible drive cages; you have to disassemble things to swap (or add) drives, and then if you actually add a second drive you need to update the BIOS. The second is that it is only a half-length server. Half length servers are an interesting response to the ceaseless shrinking of component sizes; I actually like that you can get a perfectly good 1U server into half of the space that it needed not that long ago. But they turn out to have a little drawback.
We're not replacing our existing 1U SunFires in large blocks, the way some places might; instead we're pulling them out one by one as we 'upgrade' servers to Ubuntu 12.04 (by reinstalling them on new hardware). This leaves behind little 1U holes between still in production servers. If the Dells were full-depth, it would be easy to fill these holes with the new servers. But it turns out that you can't do this with half-length servers; 1U and probably even 2U is simply not enough vertical space to give you access to the back of a half-length server (not unless you have very small hands). No plugging in power, networking, serial cables for the console, or what have you. Instead these Dell half-length servers have to go in clear areas of rack space (which, fortunately, we have enough of) that have several U of space around them.
(It's not a problem if you have a bunch of these half-length servers all in a stack, because then you have enough free space for your hands.)
A big operation would probably do something like declare periodic downtimes to compact the racks (or with sufficiently long cables and enough daring, pull and reinsert servers while they were running). We're probably just going to let things be; we'll decommission more SunFires over time anyways, so sooner or later those 1U holes will widen into bigger ones.
(At the moment decommissioned SunFires are not being thrown out but are instead being made into a spares pool for our iSCSI backends. The need for such a spares pool is really the only reason for us to decommission most of these SunFires anyways; with a few exceptions they still run fine despite being relatively old hardware by most people's standards.)
PS: I like Dell's half-length solution a lot better than the other one I saw, which was to cram two half-width servers into a single full length 1U server chassis. That struck me as just plain awkward in various ways.