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Wheezy regressions on QNAP TS-419P+: port issue?

I recently installed Debian Squeeze on a new TS-419P+, and it ran
nicely. So, to make it more interesting, I upgraded to Wheeze, and now
I'm seeing a performance/responsiveness regression whose cause I can't
quite figure out.

This has following symptoms:

- Big-file dd with large block size (over 16 kiB or so, but especially
like 1 MiB), from both RAID-5 backed files and bare disks performs
much slower (30-60%!) than before.
- dd with small blocks (4 kiB would seem to be close to sweet spot)
performs roughly as expected, but slightly slower when served from
RAID-5 (maybe 25% difference at most).
- SSH and even serial console interaction on other shells feels much
choppier than it used to be according to my memory with Squeeze while
I was doing these benchmarks.
- System is fine without this disk activity (I didn't try if network
activity would have a similar effect, but just busylooping processes
cause no trouble).

Do you have a chance to try this on Kirkwood system, preferably with
Marvell 6282, not 6281? Somehow I'm feeling that
linux-image-3.0.0-1-kirkwood kernel is missing interrupts, or
something. There is no indication of this on dmesg, though.


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