I think your observations suggest that, to a first approximation,
filling drives with bytes to the same absolute level is better for
performance than filling drives to the same percentage full. Assuming
random distribution of PGs, this would cause the smallest drives to be
as active as the largest drives.
E.g. if every drive had 1TB of data, each would be equally likely to
contain the PG of interest.
Of course, as more data was added the smallest drives could not hold
more and the larger drives become more active, but at least the smaller
drives would as active as possible.
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