Howto: Racks and rackmounting

I’m going to start a special feature on Fridays. It’s going to be sharing the sorts of tips that systems admins need to know, but can’t learn in a book. There are so many things that you learn on the job, figure out on your own, or run across on the net which make you realize that you’ve been doing something wrong for years. Sometimes you learn about things that you might have had no clue about. For instance, I just found out that you can do snapshots with LVM

Anyway, this Friday, I’m going to be showing you what I know about server racks.

I started out on a network that had a bunch of tower machines on industrial shelves; the sort you pick up at Harbor Freight or Big Lots. When we moved to racks and rackmount servers, it was like a whole new world.

The first difference is form-factor. Tower servers are usually rated by the “tower” descriptive. Full tower, half tower, mid-tower. Rack Servers are sized according to ‘U’s, short for “Rack Unit”. It’s equivalent to 1 3/4 inches, so a 2U server is 3.5” tall. The standard width for rackmount servers is 19” across. Server racks vary in depth, between 23 and 36”, with deeper being more common.

Instead of shelves for each server, rack hardware holds the server in place, usually suspended by the sides of the machine. They allow the server to slide in and out, sometimes permitting the removal of the server’s cover to access internal components. Different manufacturers have different locking mechanisms to keep the servers in place, but all rack kits I’ve seen come with instructions.

To anchor the rack hardware (also known as rails) to the rack itself, a variety of methods have been implemented. There are two main types of rack. Round hole racks, seen at the left, require a special type of rack hardware. Much more common is square hole racks, which require the use of rack nuts. The rack nuts act as screw anchors to keep the hardware in place. Some server manufacturers have created specific rack hardware that fits most square hole racks, and don’t require the use of rack nuts. Dell’s “rapidrail” system is one with which I’m very familiar. Typically you get the option of which rail system you want when you purchase the system.

Installing the rack nuts is made easier with a specialized tool. I call it the “rack tool”, but I’m sure there’s another name. The rack nut is place with the inside edge clip in place, through the hole. The tool is inserted through the hole, grabs the outside clip, and then you pull the hook towards you. This pulls the outside clip to the front of the hole, securing the nut in place.

A typical server will require eight nuts, usually at the top and bottom of each rack unit, on the right and left sides, front and back. Each rack unit consists of three square holes, and a rack nut is put in the top and bottom of both the right and the left sides. Several pieces of networking equipment have space for four screws, but I’ve found that they stay in place fine with two. I can’t really recommend it for other people, but if you’re low on rack nuts, it’s better than letting the switches just sit there (and it almost always seems like you have fewer rack nuts than you need once your rack starts growing). If you only use two screws to hold in your networking equipment, make sure it’s the bottom two. The center of gravity of a rackmount switch is always behind the screws, so if the top screws hold it up, the bottom has a tendency to swing out, and that’s not good for your rack or your hardware.

While I’m on the subject of swtches, let me give you this piece of advice. Mount your switches in the rear of the rack. It seems obvious, but you have no idea how many people mount them on the front in the beginning because “it looks cooler” and then regrets it when they continually have to run cable through the rack to the front.

Once your rack starts to fill out, heat will become an issue. When you align your rack for your air conditioner, another bit of common sense that’s frequently ignored. Air goes into the servers through the front, and hot air leaves through the back. This means that when you cool your rack, you should point the AC towards the front of your rack, not the back.

Air comes in here… And leaves back here
And leaves here...

It’s probably not a stranger to anyone who’s used a computer, but the cables seem to have a mind of their own, and nowhere is it more apparent than a reasonably full server rack. Many higher-end solutions provide built-in cable management features, such as in-cabinet runs for power cables or network cables, swing arms for cabling runs, and various places to put tie-downs.

There is no end-all-be-all advice to rack management, but there are some tips I can give you from my own experience.

Use Velcro for cabling that is likely to change in the next year. Permanent or semi-permanent cabling can deal with plastic zipties, as long as they aren’t pulled too tight, but anytime you see yourself having to clip zipties to get access to a cable, use Velcro. It’s far too easy to accidentally snip an Ethernet cable in addition to the ziptie.

Your rackmount servers will, in many cases, come with cable management arms. Ignore them. Melt them down or throw them away, but all they’ve ever done for me is block heat from escaping out the back.

Label everything. That includes both ends of the wires. Do this for all wires, even power cables (or especially power cables). Write down which servers are powered by which power sources.

If you have a lot of similar servers, label the back of the servers too. Pulling the wrong wire from the wrong server is not my idea of a good time.

Keep your rack tool in a convenient, conspicuous spot. I ran a zip tie through the side of the rack, and hang mine there.

(Some photos were courtesy of Ronnie Garciavia Flickr)

  • Jemimus

    Great post! I have also always been struck by the lack of attention this area of sysadmin is given in books and courses. I 2 books though, that appear to cover these topics a bit, that I blogged a bout here:

    Also the good old Microsoft NT4 book Networking Essentials covered at least some stuff about cables, cable types and connectors.

  • Jemimus

    Oh.. one more question, and I am probably pretty ignorant for asking this. But … what is a racktool used for? I have put dozens of racks together in my time, but never encountered this tool.

  • Matt

    The rack tool makes it much, much easier to insert and remove rack nuts.

    The idea is that you start the rack nut by putting one wing through the hole. Then you insert the rack tool through the hole and use the small curved top to pull the other nut wing through.

    Here’s a better view and you can see the little bent tip that grabs the wing:

    I’ll have to put a movie up of using one to put a nut in a rack.

    thanks for the comment!

  • Kenny

    I don’t know about you, but pulling the wrong wire from the wrong machine is the exact definition of a good time for me!

  • TooManyRacks

    Certainly a good bit of info. The only thing that I would add to it for certain is in regards to the ‘screw in’ type racks (not the square hole ones) and that is this:

    All the holes are NOT equally spaced apart. Much like the lines designate U spacing on the square holes make sure you’re putting the screws in the correct holes…

  • Matt


    That’s a great observation. I should have taken a picture of the one where I used a felt-tip to delineate the rackunits.

  • Greg

    Where was this post when I needed it!? Last year I worked at a small company that started out with a single box running their entire network. When I got funding for 4 servers and a half rack I searched the net and books for all the rack info I could find. I was fresh out of college working my first job in IT so I didn’t know much about rack setups. There isn’t a whole lot of info out there. Great post.

  • Warren Guy

    You know, I actually had no idea that “rack tools” existed! I’ve always used a flat-head screw driver or a random “PCI slot protector” from a PC case unlucky enough to be lying around ;).

  • This is a great tutorial, but I just wanted to add to this — I did a write-up on a better cage nut tool that’s out there for not all that much money. I’ve been much happier with this plier-style tool than with the strip of metal.

  • I’ve also always heard them referred to as “cage nuts”.

    First thing I always do when I get a new rack is go to and get several bags of additional nuts – as the datacenter never gives me enough to populate a cabinet…

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  • thank you very much. it helped a lot.

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