In previous tips in this series, we introduced the idea that implementing availability requires taking a layered approach, good system administrative practices, backups, disks and storage (and why larger disks aren't always better), and networking.
In Part 6, we will look at the environment around your systems, servers in particular, and see how your data center environment can affect system availability. Although there are many directions in which we could take this discussion, I want to look at the issue of cabling, and more specifically, cable ties and wraps.
There is a natural tendency to make data-center racks and cable runs as neat and organized as possible, mostly because it looks good. This leads to cables being tightly fastened with cable wraps to any appropriate surface. The really organized administrator will use cable wraps every few inches along the entire cable run to make sure that those cables are really tied down, and aren't going anywhere.
Tying down cables is a bad policy; it sacrifices recovery time for neatness. By tightly tying cables down, you can create several problems:
1) When a cable needs to be replaced, it requires opening (usually with a knife, although some more expensive cable wraps use Velcro to stay closed) all of those nice neat cable wraps to free the cable in question. Knives and critical data cables are not usually a good combination.
2) Tightly tied cables make it almost impossible to move the systems that are attached to these cables. Since the cables have no slack to them, there's no way to move systems without freeing all of the system's cables. Knives again.
3) Human nature is such that administrators are more likely to try to shift a system slightly rather than cut all those cable ties. Subtle movements can cause breaks in cables, or they can make connectors disconnect from their system. 4) When it is necessary to swap one system for another on a rack, it is very difficult to do so quickly when cables are rigidly fastened to the rack. If the system in question is in the middle of a stack of systems on a shelf, then all of the systems above it must be moved up first, and then down (to fill the space just vacated). If there is not enough slack in the cables to handle that minor move, then, once again, cable wraps must be cut.
If you do elect to use cable ties, make sure that the pressure points on the cables are not where the cables connect to their systems. If you don't leave a little slack in the cables at their connections, you risk putting undue pressure on the connectors, which can damage them, or cause them to disconnect from their systems.
Cable ties seem like a trivial issue in the grand scheme of high availability, but they most certainly is not. Improper use of cable ties, which seems to be the rule in most shops, can actually have a serious impact on availability.
Copyright 2002, Evan Marcus
Evan L. Marcus is the data-availability maven for Veritas.