cache sync on block device.

Hi,
I want to  implementing cache sync functionality for a block device. I referred to CTL code, below link says
http://freebsd.1045724.n5.nabble.com/CAM-Target-Layer-and-dev-isp-td5727548.html

" If you use a block device (like a zvol) as the backing store, you'll want 
to disable sending cache syncs to the disk, since that will trigger a GEOM 
assertion. 

ctladm realsync off"

How is cache sync (flush) implemented for a block device ? bio_cmd=BIO_FLUSH results in ASSERT.

Thanks
Sreenivas