write cache on usb disks and zfs

Hi all

I am having a few problems with external usb2 disk and write caching.

I have some caddies which allow write caching (seen and editable via 
camcontrol) and others which automatically disable it. i.e.

#camcontrol modepage da2 -m 8
IC:  0
ABPF:  0
CAP:  0
DISC:  0
SIZE:  0
WCE:  0
MF:  0
RCD:  0
Demand Retention Priority:  0
Write Retention Priority:  0
Disable Pre-fetch Transfer Length:  0
Minimum Pre-fetch:  0
Maximum Pre-fetch:  0
Maximum Pre-fetch Ceiling:  0

and

#camcontrol modepage da2 -m 8 -e
camcontrol: no editable entries


I am trying to use the USB drives for backup and would like to use zfs on 
them, but the problem I have is with the cache disabled, I get lots of

kernel: (da2:umass-sim0:0:0:0): SYNCHRONIZE CACHE. CDB: 35 0 0 0 0 0 0 0 0 0
kernel: (da2:umass-sim0:0:0:0): CAM Status: SCSI Status Error
kernel: (da2:umass-sim0:0:0:0): SCSI Status: Check Condition
kernel: (da2:umass-sim0:0:0:0): ILLEGAL REQUEST asc:20,0
kernel: (da2:umass-sim0:0:0:0): Invalid command operation code
kernel: (da2:umass-sim0:0:0:0): Unretryable error

errors in /var/log/messages.

I am assuming that the error messages are harmless, but it would be nice for 
zfs to pay attention and not continually issue the commands.  Is there a way 
to do this?


Duncan