MTBF and Software

Like many people, I sometimes need a bonk on the noggin to remember some essential bit of wisdom that I shouldn’t have forgotten in the first place. Such is the case with the relationship between hardware and software. In many cases, developers have lost their connection with the hardware. Even though it seems quite obvious that the software provides instructions that change the state of the hardware, developers don’t really seem to make the connection. Once you remember the hardware connection, it also begins to make sense that any aberration of the functionality of that hardware will also reflect in the reliability of the software. In short, the Mean Time Between Failures (MTBF) of the hardware also has an effect on the software that runs on the hardware and causes the hardware to perform specific tasks.

The issue that drove the point home for me is a simple hard drive. This particular hard drive came with the system and the vendor used a lower cost drive to keep prices low (normally I get really high quality hardware simply to avoid problems). What this means is that the MTBF of the drive is also quite low. Unfortunately, I encountered the MTBF late last week as a glitch that caused me to think there was a problem with my software. The software was just fineā€”it was the glitch with the hard drive that was the source of the problem. I only realized this fact after testing the software on another system. (Unfortunately, the hard drive got worse and took some of my system configuration with it, but I maintain backups, so the loss was minimal.)

However, the partial failure of the drive caused me to realize yet again that software can only operate correctly when the underlying hardware also operates correctly. I can’t remember the last time I read anything that even broached the topic of hardware as a potential source of software problems. It makes me think that there are probably developers out there right now trying to find the error in a piece of software that doesn’t even exist in the software, but is a matter of some hardware glitch.

It’s important to realize that hardware doesn’t always fail in a predictable manner either. For example, a glitch can occur when a hairline fracture occurs in the runs of a board. This sort of error makes its appearance when you start the system. When the board heats up, the failure goes away because the breach in the run is sealed. Expansion of the metal fixes the problem. I’ve actually encountered a host of incredibly odd hardware problems over the years, many of which could appear as an isolated software issue given the right circumstances.

The lesson relearned in this case is to always test software on multiple systems. It’s essential that these systems use different components. Doing so will eliminate a number of non-software issues as the source of a problem. For example, using mismatched systems can help you understand when an error is due to a particular device driver. The point is that you need to avoid shooting yourself in the foot by not thinking of all the possibilities. Complex software interacts with the hardware in a complex way, which makes it all the more likely that some insignificant hardware or firmware issue will cause you woe as a developer.

What are your experiences with odd hardware- or firmware-related behaviors? Have you even encountered such behaviors? Let me know at


Review of LED Christmas Lights

I’m always looking for ways to make self-sufficiency pay. One of those methods is to do more with less. In my CFLs for Free post, I described how you could purchase just one Compact Fluorescent Light (CFL) and eventually obtain a house full of them (using the money saved to buy new bulbs) that would end up saving you a lot of money. Thinking carefully about new technologies and how they can make you more self-sufficient is a good way to keep more money in your pocket.

Not all new technologies end up saving you money. The Light Emitting Diode (LED) is one of them. Yes, the new flashlights are fantastic and I absolutely love the one I own. It puts out an immense amount of light seemingly forever on a single change of batteries. However, LED Christmas tree lights are another story. It would initially appear that they’d be a winner. Their life expectancy is supposedly much longer than standard bulbs and an entire 200 light string consumes a miserly 9 watts when you buy one of the nicer sets. In addition, the light they produce is vibrant.

Unfortunately, the longevity of LED Christmas lights is a problem. Out of eight test sets I initially purchased for testing, one set is completely dark and two others have dark segments in just one year of use. Of course, the problem is likely with just one bulb in each darkened segment. However, this is where the another problem occurs, the bulbs are glued into place and you can’t change them. (A few newer sets do include replaceable bulbs, but each vendor appears to have a different socket scheme so the bulbs from one vendor aren’t interchangeable with those of another vendor.) However, whenever you can get them, get the sets with replaceable bulbs.

Having read the vendor documentation carefully, I had anticipated a problem or two. The first thing you need to know is that the vendor is misleading you about the longevity. The sets I reviewed specified bulb life between 25,000 and 100,000 hours. The term you need to know here is Mean Time Between Failures (MTBF). MTBF is a statistical measure that doesn’t really tell you anything unless you know how to apply it. An MTBF of 25,000 hours means that half of the bulbs will last that long and the other half will last longer. There is an actual mathematical probability curve that specifies bulb reliability, with higher failure rates at the beginning and end of the curve and a relatively stable period between (called a bathtub curve). You also need to consider what MTBF infers. The vendor is telling you the life expectancy of any given bulb. In order to find the bulb life expectancy of the light set, you divide the individual bulb life by the number of bulbs in the set. A 200 light set that has bulbs with an MTBF of 100,000 will, on average, deliver only 500 hours of light without failure. If you’re like most people who turn their lights on at 6 pm and turn them off at 11 pm, the set will last, on average, 83 days or about 3 years. As a consequence, it doesn’t surprise me that three out of eight light sets have problems.

The second thing you need to know is that LED lights are polarized and of different characteristics. If you’re used to working with miniature incandescent sets with replaceable bulbs, you know you can slip a bulb out of a non-conforming holder and put it into a conforming holder without problem as long as the bulb is of the right type for the number of lights in the set. In addition, it doesn’t truly matter how you insert the incandescent bulb as long as the two leads stick out appropriately. Not so with LED bulbs. It’s possible to put them into the holder backward because there is a positive and negative end. In addition, a check of those replaceable bulbs show that some have resisters attached to the bulbs and others don’t. In other words, you must get the replacement bulbs for your set from the vendor who produced your light set. The addition of replaceable bulbs is a good step forward in LED technology, but things are still too complicated for most people to handle.

There is actually a way around these repair issues and that you can salvage your LED Christmas lights when they fail. On Thursday I plan to review LED Keeper, a product I found for fixing problem light sets. The solution isn’t perfect and it does require some electrical knowledge on the part of the user, but it actually works quite well. I managed to salvage my failed light sets using this tool.

The third thing you need to know is that LED Christmas lights won’t actually save you any money unless you can buy them on sale. The problem is the high initial cost of the light set and the small amount of time you use them. The cost of running a 50 light incandescent set 6 hours per day for the 30 days that most people use them is ((20.4 watts * 6 hours/day * 30 days) / 1000) * Your Electrical Rate ($0.111362/Kwh in my case) or about $0.41. The cost of running a comparable 50 light LED set is ((4.8 watts * 6 hours/day * 30 days) / 1000) * 0.111362/Kwh or about $0.10. The savings of $0.31 per year isn’t very large when you consider the difference in light set cost of about $21.00. The lights would have to work for 68 years to pay back your investment. In order to make LED Christmas lights work as an investment, you have to buy them on sale. However, you might simply like the fact that they produce such vivid colors that the cost differential isn’t a concern.

Overall, I can’t recommend LED Christmas lights as they currently exist. The vendors aren’t being honest about how long they last, most sets are impossible to repair, and even when the set is repairable, the replacement lights aren’t standardized. Adding insult to injury, you’re paying a much higher price for these sets. LED Christmas lights are getting closer to being a bargain each year though and it’s likely that most people will be able to start benefiting from them in a few more years.