Linux, politics, and other interesting things
Almost two years ago I blogged about a strange performance problem with SATA disks . The problem was that certain regions of a disk gave poor linear read performance on some machines, but performed well on machines which appeared to be identical. I discovered what the problem was shortly after that but was prevented from disclosing the solution due to an SGI NDA. The fact that SGI now no longer exists as a separate company decreases my obligations under the NDA. The fact that the sysadmins of the University of Toronto published all the most important data entirely removes my obligations in this regard .
In their Wiki they write “after SGI installed rubber grommits around the 5 or 6 tiny fans in the xe210 nodes, the read and write plots now look like” and then some graphs showing good disk performance appear.
The problem was that a certain brand and model of disk was particularly sensitive to vibrations. When that model of disk was installed in some machines then the vibrations would interfere with disk reads. It seems that there was some sort of harmonic frequency between the vibration of the disk and that of the cooling fans which explains why some sections of the disk were read slowly and some gave normal performance (my previous post has the graphs which show a pattern). Some other servers of the same make and model didn’t have that problem, so it seemed that some slight manufacturing differences in the machines determined whether the vibration would affect the disk performance.
One thing that I’ve been meaning to do is to test the performance of disks while being vibrated. I was thinking of getting a large bass speaker, a powerful amplifier, and using the sound hardware in a PC to produce a range of frequencies. Then having the hard disk securely attached to a piece of plywood which would be in front of the speaker. But as I haven’t had time to do this over the last couple of years it seems unlikely that I will do it any time soon. Hopefully this blog post will inspire someone to do such tests. One thing to note if you want to do this is that it’s quite likely to damage the speaker, powerful bass sounds that are sustained can melt parts of the coil in a speaker. So buy a speaker second-hand.
If someone in my region (Melbourne) wants to try this then I can donate some old IDE disks. I can offer advice on how to run the tests for anyone who is interested.
Also it’s worth considering that systems which make less noise might deliver better performance.
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