Solved Is there a simple way to disable write cache?

From ada(4):

         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).