flakey target isn't flaking

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Ubuntu's linux kernel version 3.13.0-39-generic, lvm 2.02.98.

Table:

flakey: 0 20480 flakey 7:0 0 10 1 0

That should be throwing IO errors for one out of every 11 seconds, but I run:

while :; do sudo dd bs=65536 count=1 if=/dev/mapper/flakey of=/dev/null iflag=direct ; sleep 0.2 ; done

And it never sees any errors. What gives?

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Try writes not reads.
The documentation is unclear.
Or work out a sensible way to enhance it to do more little things.

Alasdair