Say Y here if you want to be able to trace the block layer actions
on a given queue. Tracing allows you to see any traffic happening
on a block device queue. For more information (and the userspace
support tools needed), fetch the blktrace tools from:

git://git.kernel.dk/blktrace.git

Tracing also is possible using the ftrace interface, e.g.:

echo 1 > /sys/block/sda/sda1/trace/enable
echo blk > /sys/kernel/debug/tracing/current_tracer
cat /sys/kernel/debug/tracing/trace_pipe

If unsure, say N.

source code:

kernel/trace/blktrace.c