Friday, April 18, 2014

Well, I Thought I Had a Spare Drive

But it turns out that my spares collection is FUBAR. Here's the thing. I was planning on rebuilding a lab system this weekend. I have always enjoyed messing with hardware of one sort or another, I've built many systems over the years, and I have a bit of an ingrained work flow optimized for minimal SOHO office disruption, etc. Move that pile of books out of the corner and re-shelve, fill in the space with the parts collection, etc.

The parts collection was where I ran into a bit of snag.

I get a bit uncomfortable when I am down to a single spare general-purpose SATA drive. For RAID, things are a bit more complex, as I like matching firmware versions, and I also like to keep a pristine portable drive around. What I have is a new factor, in that I can't use a basic 1 TB Caviar Black. It's been sitting for several years, still in original packing. The only thing I've ever done regarding this drive was to print the output of uuidgen(1) and purchase details, and stick it into the bag. Due to the nature of my work, I need to unambiguously track which hard drives are used by which systems. Which is a topic for another post.

Now, this drive has to be sequestered, because I seem to have built up a collection of performance notes which involved I/O to an identical drive. I may want to revisit those numbers, well after the normal life of a drive, and it is worth it, to me, to remove a variable from any comparison I might care to make. An opportunity to reduce the complexity of a future data analysis, for the cost of a consumer hard drive, is a no-brainer.

So I'm down to one consumer hard drive. If I use it, I will have no spare until a replacement arrives. That isn't something I would do, except in an emergency, which this isn't. I could just drive into town, buy one, and get on with the job. But it is probably wiser to buy three online; the odds of getting identical firmware are good, so I get more down-the-road flexibility. I could make a reasonable case for buying six, and may do that.

That's a bit of a bummer, as I did want to get that hardware upgrade done this weekend. If, for whatever reason, it can't happen next weekend, then it is no longer a casual, do it ahead of the need thing. It rises to the level of Important. Murphy is alive and well, and all it will take is a call from a client.

This sort of thing is best avoided, and that it happened in this case is my own FUBAR. I failed to allow for all the factors that might influence the effective size of the spares pool. That informal work flow bit me, and better systems administrators than I can now commence laughing. Lesson learned, and I hope that others do not make the same mistake.

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