Just record how to use debugfs, tracing block IO.
Vivek Goyal said on maillist:
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You can try using tracing functionality.
- mount -t debugfs none /sys/kernel/debug
- Enable tracing on the disk you are doing IO to.
echo 1 > /sys/block/sda/trace/enable
- Enable block traces
echo blk > /sys/kernel/debug/tracing/
current_tracer
- cat /sys/kernel/debug/tracing/trace_pipe > /tmp/trace_output
- cat /sys/kernel/debug/tracing/trace_pipe > /tmp/trace_output
Let it run for few seconds. Interrupt and kill cat process.
/tmp/trace_output should have useful tracing info.