When trying to read scratched or damaged CDs and DVDs, the default
mechanism is sub-optimal. Programs like ddrescue do much better if
you turn off retries entirely, since their algorithms are designed
scan big areas fast, then winnow the areas down. Turning off retries
speeds these programs up by as much as 20x, since the drive is able to
'stream past' many small errors...
The sysctl/tunable kern.cam.cd.retry_count controls this. That
defaults to '4' (for a total of 5 attempts). Setting to 0 turns off
all retry attempts.
Reviewed by: scottl@
8550170477