What happened to disk performance in 2.6.39

Plugging or not, I'm pretty sure there are still queues involved just the same. Reading the other links in this article, it seems that plugging goes away as soon as the system determines that it has any work in its internal queues to do, therefore it's strictly a "first request" optimization. In any case, it doesn't seem to improve throughput (because it gets disabled) and worsens latency (because it delays first request service time), so it sounds useless to me in every case.

Disk schedulers already use their own variant of fair queueing, afaik CFQ gives all processes their chance to do some disk transaction when it comes their turn, in this being fairly similar to SFQ which arranges outbound network traffic into number of pre-existing queues, submitting the head element of each queue in turn (if any), giving all flows a fairly equal chance to progress.