An open question: part uniformity versus unit cost
We're in the process of renewing the hardware for our fileserver infrastructure and at this point we've basically settled on the motherboard we'll use for both the iSCSI backends and the fileservers and the hardware for the iSCSI backends; the backends will be built around a 3U case with 16x 3.5" drive bays on the front and 2x 2.5" drive bays on the back (and dual power supplies and various other bits). As we were discussing things today, we wound up asking a question: does it make sense to specify out a different case for the fileservers?
Because they don't need the drive count of iSCSI backends, the fileservers don't need a 3U case; they could certainly fit in a 2U case without problems, even with room for the hack of local L2ARC SSDs. Based on some preliminary investigation I just did, using a 2U case instead of a backend 3U case could save us perhaps $200 (maybe more, maybe less, it's hard to tell). This is not pocket change to us; if nothing else, that would buy that much more RAM for each fileserver.
But there are two advantages of using the current 3U case for fileservers as well as backends. First, it means that we don't have to search out, specify out, price out, and probably get in an initial evaluation unit of a 2U fileserver setup. Instead we can simply buy, right now, as many servers as we want. This points out the second advantage: parts uniformity. If the only hardware difference between backends and fileservers is how much memory they have and maybe what expansion network card they have, we simplify our lives as far as parts and spares go. We're only buying and deploying one thing instead of two and our spares become more flexible and possibly cheaper.
(There's also extra flexibility that we're unlikely to ever use, for example we'd have the ability to build a low(er)-cost ZFS fileserver that uses local disks instead of iSCSI at the cost of no failover and so on.)
I honestly don't know if this parts uniformity is worth the cost. It's attractive, though (partly because I've already spent enough of the past few months trying to get vendors to talk to me and take our money).
(True parts uniformity would require more than just the case, we'd have to throw in an extra SAS card for each fileserver.)