Add corrupt_bio_byte feature to simulate corruption by overwriting a byte at a
specified position with a specified value during intervals when the device is
"down".

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

Add 'drop_writes' option to drop writes silently while the
device is 'down'.  Reads are not touched.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

This target is the same as the linear target except that it returns I/O
errors periodically.  It's been found useful in simulating failing
devices for testing purposes.

I needed a dm target to do some failure testing on btrfs's raid code, and
Mike pointed me at this.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>