Just to add to the clutch here, also check your bdflush settings (if you're on linux) or equivalent (if you're not.)
Many times the swapping algo in linux can be quite bursty if you have it set to move too many pages at a time during cleanup / flush.
According to vmstat it doesn't swap near the spikes, so I don't think this is the problem. I posted a vmstat sample in another reply, where you can see an example of what happens.
Regards, Anders K. Pedersen
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