Tuesday, 02 October 2007

Sarah had been complaining that MythTV hadn't recorded an episode of The Simpsons in months. It was indeed strange, as you could see that it was going to record episodes if you looked at the upcoming recordings.

The other night we happened to be watching some recorded TV around the time an episode was scheduled to be recorded, and sure enough, it started recording one.

When we finished watching what we were watching, the episode had vanished. I didn't think I'd seen anything relevant to auto-expiry in the logs for the backend.

A few days later, another episode was recorded. We even watched it. It disappeared afterwards.

I was sufficiently curious at this point as to what was going on. It turned out that way back in the distant past when I set up MythTV, I misunderstood what "recording groups" were. When I set up recordings of throw-away things like the news, I created a recording group called "Short retention", assuming I could go somewhere else and make stuff in that group get expired quicker than stuff in the "Default" recording group.

(Incidentally, I only in the last few months discovered the veritable cornucopia of things that you can do from the recordings listing screen when you press the Menu/M button on the remote/keyboard. The recording groups now make much more sense)

I think there's the word "storage" somewhere in the user-interface around this area, which may have been what confused me. Turns out that I never went and tried to set this mythical expiry policy up, because that's not how auto-expiry works. The choices (as of 0.20) are "Oldest show first", "Lowest priority first" and some sort of weighted combination of the two.

It turns out that I had it configured for "Lowest priority first", and The Simpsons had the lowest priority of all TV shows, so that it'd always lose out in the event of any scheduling conflicts.

The SAN's perpetually full, so it's always auto-expiring stuff, and The Simpsons are what get expired first by this policy. Kinda funny really.

So I switched the policy around to the weight combination of priority and age, and now it's going to auto-expire recordings from a year ago. Hooray.

Today I received the replacement hard drives for the two that failed the other week.

Despite me not ticking the box (or paying the $10 for the upgrade), they gave me 500Gb drives to replace the 400Gb drives that I'd shipped them. I was a little dismayed to find that they were labelled as refurbished drives though. Oh well.

I plugged the first drive in, and it started exhibiting the same behaviour as the one that had failed, and I immediately thought "oh, they've shipped me a dud refurbished drive". So I tried the other one. It also did the same thing. I started to smell a rat.

I have a slightly convoluted power setup for these four disks. They're all connected to minotaur via IDE-USB adapters, and they have external DC power supplies. They're just sitting on top of minotaur, which is a Pentium III 1-RU server, in the bottom of my linen cupboard. In order to reduce the number of plugs involved, I have an IEC Y-cable, which goes to two of these DC power supplies (which take an IEC cable in one end and has a Molex connector on the other end), and the Molex cable from the DC power supply has a Molex splitter cable on it. So I'm powering two drives from each DC power supply, and each DC power supply is connected to an IEC Y-cable.

There's no redundancy whatsoever, and I'm not that concerned. I don't have a UPS anyway, and the whole thing has worked fine for months, and it reduces the rats nest of cables a little bit, which is good. I already have way too many things plugged into my linen cupboard.

Anyway, so after both new drives appeared to be faulty, I discovered that both drives that had failed within a day of each other were connected to the same DC power supply.

The plot thickened.

So I dug out a standalone IDE-USB hard drive enclosure (with a separate power supply) and tried that. Everything worked. I tried it with just the enclosure's power supply and either of the IDE-USB adapters I'd been using previously. Everything worked. So I'd managed to narrow it down to the power supply. Most unusual.

So fortunately I have four of these DC power supplies, as they came with the IDE-USB adapters, and I freed up two of them when I went for the aforementioned crazy power splitting setup, so I dug out one of the two spares, and tried that, and everything worked. So I've no idea why the power supply decided to go dicky after months of working. I've yet to introduce my multimeter to it to try and work out what's going on.

I'm a bit embarrassed that I didn't do more homework before I declared the drives to be faulty. To be honest, they were clicking and generally looking "failed", and I've had a lot going on lately, so I just jumped to the conclusion. Oh well, at least I still haven't lost any data. Too bad I have 200 Gb that is going to go to waste.