…he GEOM

disk layer by default returns ENOTSUPP in response to a BIO_FLUSH command.

On AWS, local volumes do not advertise themselves as having write-cache enabled.
When they are selected for L3 on all HDD nodes, the L3 subsystem may inadvertently
kick these L3 devices if a BIO_FLUSH command fails with an ENOTSUPP return code.
The fix is to make GEOM disk return success (0) when this condition occurs and
add a sysctl to make this error handling config-driven