> > i often do dd if=/dev/rcd0d of=file bs=2k to take CD image
> >
> > in 1.5.2 i was normally using machine while doing it. now i can't.
>
> I still can do that, but it's because my IDE disk is fast, I guess.
> disk is only 10% busy.

yes.

> >
> > if so - apps that doesn't use disk at all for sure (like text editor)
> > should work fine. but it isn't - delays are small but noticable (up to 1
> > second).
>
> it looks like file activity is still pushing out some data from RAM,
> even if it should not.

some other possible explanation.

some kernel code uses almost 100% CPU in such case.

the disk request sorting procedure may be this - it would sort HUGE
request list which will take time leaving almost none of CPU time for user
process.

no such effect will be seen if disk is not 100% loaded - such case req.
queue will never go so long.