I'd just like to take one step back here for the reading problem - let's keep it simple. When we worked with DDN 8500 controllers we established that with something like 10 reading threads reading 1MB chunks randomly from OSTs we achieved the full bandwidth of the array. Read-ahead probably only damages things at this point (but is very important for reading smaller files).
There are many users of the 9500 SATA arrays now, and it surprises me that we don't have the OST survey graphs for these arrays. I believe that someone from our staff is at work with Sandia to get one done. This should show us the parameters to get full bandwidth at least when the application reads at least X MB, where X is hopefully still 1 (if not we have a nasty networking problem on our hands).
I would say that "playing" with the parameters (DDN settings, Lustre read ahead etc) is almost certain to make things worse in these use cases, and be highly confusing. Perhaps we should disable at least the Lustre read-ahead stuff when we have the minimum sizes to reach full bandwidth.
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