Poor SCSI disk preformance

[Dan Nelson]

In the last episode (Jan 06), Derek Marcotte said:
> Actually, just for kicks:
> 
> # dd if=/dev/da0 of=/dev/null bs=128k &
> [1] 1839
> # iostat -K -w 1 da0
>       tty             da0             cpu
>  tin tout  KB/t tps  MB/s  us ni sy in id
>    1   42 64.00 607 37.92   1  0  1  0 98
>    0   43 64.00 222 13.87   0  0  2  0 98
>    0   43 64.00 223 13.92   0  0  0  2 98
>    0   42 64.00 224 13.98   0  0  2  0 98
>    0   43 64.00 222 13.86   0  0  3  0 97
>    0   43 64.00 223 13.92   0  0  1  2 98
>    0   43 64.00 223 13.92   0  0  2  1 97
>    0   42 64.00 223 13.92   0  0  3  0 97
>    0   43 64.00 223 13.92   0  0  1  0 99
> 
> Seems to give me the performance that I expect...

Aha.  Check the WCE bit to see if your write cache is enabled on the
disk:

# camcontrol mode da0 -m 8 | grep WCE

If it's not set, that could be contributing to the speed difference
between reads and writes.  Set it by running "cmcontrol mode da0 -m 8
-e -P 2", and set "WCE: 1".

-- 
	Dan Nelson
	dnelson at allantgroup.com