Yes, you need rails, but make sure you confirm with your colocation provider which rails you actually need.
Dell sells 2 types, one for square bracket (aka Rapid Rail), and one for round hole. Plus some non-standard Racks may not even work with either of the two rail kits Dell offers, so it's always best to confirm with your colo what you need.
I am in process of ordering some 20 servers, but I think i missed the point about rails. Do I need rails for cabinet? or rails are only for racks?
Just to make things clear, I will be buying 1U/2U PowerEdge Servers from DELL, and colocating them in cabinet but I am not sure if cabinet require rails or they have something different .
Neither cabinets or racks *require* rails physically, but they are recommended. Rails make it easier to access a server for repairs (just slide it out and work on it). If you don't have rails, what you would do is put a shelf underneath every 5 or so servers, depending on the load bearing capacity of the shelf (rack mount shelves do anywhere from 25 lbs - 200 lbs). Then, to take out a server in the middle, you have the nasty job of taking the server out while having to lower all the servers on top of it. Or, you can do what I do, and go to Home Depot and have them cut you 1.75" spacers out of 2x4's, then use this to support the servers on top until have replaced the server you took out. Either solution is not ideal, but at least the latter makes it a bit easier.
Ultimately, ask your colo provider if they require you to have rails. Most likely they won't, but they'll say what I just told you, "we recommend it"
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If you're sending those servers to a colo facility definitely get rails. THose systems are heavy and large. There is no way to simply front mount those things and if you don't include rails the colo provider may be forced to find some unusual way to mount them, etc. and that may not always be good for the long run.
Dell rails are pricey $139 off the website I believe but they do have a nice fat margin on those so it's often a negotiable item with your purchase.
I haven't checked ebay recently on 1850, 2850 series rails, but the last couple of years you could often find Dell 1750, 2650, etc rails anywhere from $40->90 plus shipping.. so sometimes there was a little savings to be had. Might even be able to call in to check on refurb/returns to see if they'll cut you a deal on rails like they do on other parts (as much as 25->50% off).
I really don't see the value in skimping on not buying rails, I'd be too concerned about my equipment if it was colo'd somewhere without them and not properly secured, not to mention a company that allowed other customers to do that above or below my equipment.