These variables determine whether device read-ahead and write caches
should be enabled globally or per-device or disabled. Set to 1 to
enable write cache, 0 to disable, -1 to leave it as-is. Values modi-
fied at runtime take effect only after device reset (using the reset
subcommand of camcontrol(8)). Because of that, this setting should
be changed in /boot/loader.conf instead of /etc/sysctl.conf. The
global default is currently 1. The per-device default is to leave it
as-is (follow global setting).