I've noticed that when I do heavy write applications, the whole system slows down. To test this further I ran this to do a (relatively) low-CPU, high disk activity:
john -incremental > file_on_SSD
This pumps out tens of thousands of strings per second to a file on my system disk.
When it's doing this, the mouse lags, TTYs become unresponsive, applications "fade" and generally the whole computer becomes unusable. When I can eventually Control+C
john, the system comes back to full strength after a few seconds.
This is an extreme example but I have similar issues with slightly less write-intensive activities like copying big files from fast sources or transcoding.
My main OS disk is a quite fast SSD (OCZ Agility 60GB) with EXT4. If I write
john's output to a mechanical disk with EXT4, I don't experience the same slow-downs though the rate is a lot slower (SSD does ~42,000 words per second, mechanical does 8,000 w/s). The throughput may be relevant. The mechanical disk also has nothing to do with the system. It's just data.
And I'm using kernel 2.6.35-2 but I've noticed this issue since I got this SSD when I was probably using .31 or something from around that time.
So what's causing the slowdown? EXT4 issue? Kernel issue? SSD issue? All of the above? Something else?
If you think I need to run an additional test, just drop a comment telling me what to do and I'll append the result to the question.