Debugging system freezes on filesystem writes

[Marcus Sundman <marcus@hibox.fi>]

Hi,

I have a big problem with the system freezing and would appreciate any 
help on debugging this and pinpointing where exactly the problem is, so 
it could be fixed.

So, whenever I write to the disk the system comes to a crawl or freezes 
altogether. This happens even when the writing processes are running on 
nice '19' and ionice 'idle'. (E.g. a 10 second compile could freeze the 
system for several minutes, rendering the computer pretty much unusable 
for anything interesting.)

Here you can see a 20 second gap even in superhigh priority:
# nice -n -20 ionice -c1 iostat -t -m -d -x 1 > http://pastebin.com/j5qnh2VV

I'm currently running 3.5.0-17-lowlatency on the ZenBook UX31E, using 
the NOOP I/O scheduler on the SanDisk SSD U100. The chipset seems to be 
Intel QS67. I've had this same problem on 3.2.0 generic and lowlatency 
kernels.

The syslog says nothing relevant before/during/after these freezes.

Any ideas on finding the culprit?


Regards,
Marcus