block_dump enables block I/O debugging when set to a nonzero value

When this flag is set, Linux reports all disk read and write
operations that take place, and all block dirtyings done to
files. This makes it possible to debug why a disk needs to
spin up, and to increase battery life even more. The output of
block_dump is written to the kernel output, and it can be
retrieved using "dmesg". When you use block_dump and your kernel
logging level also includes kernel debugging messages, you
probably want to turn off klogd, otherwise the output of block_dump
will be logged, causing disk activity that is not normally there.

$cat /proc/sys/vm/block_dump
$echo 1 > /proc/sys/vm/block_dump
$cat /proc/sys/vm/block_dump