They are not completely free of cost when disabled and
when enabled emitting debug output for every command
submitted produces far too much output to be useful.
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com> Reviewed-by: Mike Miller <michael.miller@canonical.com> Reviewed-by: Webb Scales <webb.scales@hp.com> Signed-off-by: Christoph Hellwig <hch@lst.de>