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As the densities increase (say 2x every couple of years) you can replace two spindles with one spindle, etc.

In the system with 6 10,000 rpm Ultra/320 LVD SCSI hard drives, I had 4 of the drives rather small: 17 GBytes each. The other two were the same, but 76 GBytes. That was all the space I needed. It is true that I could have purchased a single drive with that capacity. But I did not want that. I was running database applications that before were on two hard drives and they took 8 to 12 hours to run. So I built the new machine with the four small extra drives that were dedicated to the database application. That cut those jobs down to two hours or less each.

They were not set up as RAID, because I wanted to control where the data were kept to minimize seek times. With raid, that could work out, but mainly for systems where there are a lot of different applications working, and where a dedicated hardware RAID controller with enough memory for buffering and a smart processor of its own to manage all that were there too. Software raid would really not deal with that kind of think. At least not back in 2003 when I designed that system.

While it is true that that machine had processors about 6x faster than the other, the jobs were largely IO limited, so processor speeds did not matter all that much. Old machine had two Pentium III 550 MHz processors. New machine had two 3.06 GHz hyperthreaded Xeon processors, so they pretended to be four processors. The reason I did that was precisely to get more spindles so the dbms did not have to do so many seeks, which were the problem for my workload. If the index for a table was on one drive, I put the corresponding to that index on a different drive. That way each head assembly could stay relatively close to where it needed to be instead of seeking back and forth. Those two 13 year old drives are still running fine on my old computer. I do not really need that machine anymore, but since it is there and works just fine, I keep it. Came in handy when the new machine died after Sandy came through here. So why junk it.

The so called new computer is now history, having been replaced by a Dell T7600. Since I no longer do that kind of database work, I do not have all those SCSI drives on the new machine. I do have the 350 GByte drive that came with it, the 1 TByte one I added to put Linux on it, and another 1 TByte drive I do not actually need, but for $70, including shipping, I could not resist. I have room for one more of those SATA drives, but I cannot imagine getting another one, even for $10.
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