Bug 470736

Note that even with pulseaudio running under SCHED_FIFO at priority 14, so its unlikely that the issue is waiting for CPU time.

The system has to write the data that you are copying to the external disk. It can't just leave it in memory forever.

If you disable pulseaudio, does the skipping go away?

So these pages are being written to the external disk? I was under the impression that except in the case of mmap, a page being written to disk would be going to swap. Is this not the case?

It seems to me that writing to the external drive shouldn't contribute to the overall system latency. No processes are blocking on reading from the drive, data is only being written. Is this just a symptom of the high CPU requirement of bulk USB transfers?

I'll need to go back home to see if pulseaudio is contributing.

Any data written to disk shows up as page writes. And USB storage has a lot of overhead for each write.

Alright, I guess that the USB overhead doesn't really surprise me. Somehow, I get a maximum of 10 MiB/sec to/from the external drive so clearly USB transactions aren't cheap.

I'll check the effect of pulseaudio when I get home.

This bug appears to have been reported against 'rawhide' during the Fedora 10 development cycle.
Changing version to '10'.

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Ahh hah! I just found kernel.org bug #7372 (Unusable system (ie slow) when copying large files) well looking for a completely unrelated bug. It seems like this has been a known issue since 2.6.18.

Also, kernel.org bug #8636 (http://bugzilla.kernel.org/show_bug.cgi?id=8636) seems to relate to this issue. Both #7372 and #8636 suggest that the problem lies in the IO scheduler.

I opened a new kernel.org bug, #12309 ( http://bugzilla.kernel.org/show_bug.cgi?id=12309 ), to replace #7372. Hopefully this one will be a little more productive.

You can try to work around this by running this command at boot time (after all the filesystems are mounted including the USB drive):

 pgrep kjournald | xargs -n 1 ionice -c1 -n7 -p

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