Well, read errors off of a USB drive.
Still you'd think that MC'd quit trying eventually.
I think I've found the cause of this and it may indeed be a USB bus issue, not a power issue. As I mentioned this occurred on a powered USB hub and I wasn't getting power warnings from the Pi at the time.
I have been running TOP & vmstat and watching CPU/RAM/HDD and interrupt workload and the CPU, RAM and HDD have not hit any bottlenecks, it appears to be a USB bus problem, which I now find after more research, is well known about.
The USB bus in the Pi is a bit funny when you push the USB bandwidth on it because of the high polling interrupts it uses. There's several threads over in the Pi forum about this.
"The Foundation has discovered that the (USB) controller and its driver expect realtime response from the ARM core, and if Linux’s non-realtime scheduling doesn’t respond in 1 ms, a split transaction USB event can be dropped. Not surprisingly, this occurs regularly and produces lost mouse clicks, stuck keyboard keys, lost data, etc.."
As anything plugged in uses the USB bus including the onboard Ethernet port you can get into a bit of a problem with real-time audio applications with USB DACs struggling when your using high bitrate content from a USB HDD. This combined with wifi/Ethernet and other USB peripherals plugged in is what caused the crash. Essentially the USB bus crashed because it got flooded. There were thousands of those entries in my log, I just snipped a section of it.
If my understanding is correct, they tried to slow down the USB bus interrupts with some optional cmd switches but it appears that with a high workload on the bus that it may "miss" interrupt requests.
I've been playing with the optional switches to try stop the buffering and will report back what I find.
These are the /boot/cmdline.txt entries Im going to play with. (if you do, backup your SDcard first!)
dwc_otg.speed=0 -> 1 will limit USB speed to full speed 12Mbps (USB 1.1)
dwc_otg.lpm_enable=0 -> 0 by default, it disalbes LPM support - DONT CHANGE THIS
dwc_otg.fiq_fix_enable=1 -> 1 (default now) give about 10% extra performance to ARM Pi B when USB is not busy by lowering the number of interrupts USB does
dwc_otg.microframe_schedule=1 -> 1 (default now) This should fix the error when too many periodic endpoints are present
dwc_otg.nak_holdoff_enable=1 -> 1 (default now) NAK holdoff scheme, This slows down network traffic and smooths the packet delivery so memory doesn't get flooded.
So far turning this off > dwc_otg.fiq_fix_enable -> 0 has reduced buffering pauses on 192k/24bit transcoding to 96/24 but not quite eliminated it.
There's also some good info in here that I may try out. I had already tried playing with process priorities and not made any progress but there's some ideas in here.
http://wiki.linuxaudio.org/wiki/raspberrypiPS. Im also going to try the latest 3.19 kernel as well.
Someones also compiled a real-time kernel im going to try too.
http://www.emlid.com/raspberry-pi-real-time-kernel/