To enable the write cache you can use the format(1M) command:

i_like_corruption# format -e
Searching for disks...done


AVAILABLE DISK SELECTIONS:
       0. c0t0d0 
          /pci@1c,600000/scsi@2/sd@0,0
       1. c0t1d0 
          /pci@1c,600000/scsi@2/sd@1,0
Specify disk (enter its number): 1
selecting c0t1d0
[disk formatted]
/dev/dsk/c0t1d0s0 is in use by zpool zfs_tar. Please see zpool(1M).


FORMAT MENU:
        disk       - select a disk
        type       - select (define) a disk type
        partition  - select (define) a partition table
        current    - describe the current disk
        format     - format and analyze the disk
        repair     - repair a defective sector
        label      - write label to the disk
        analyze    - surface analysis
        defect     - defect list management
        backup     - search for backup labels
        verify     - read and display labels
        inquiry    - show vendor, product and revision
        scsi       - independent SCSI mode selects
        cache      - enable, disable or query SCSI disk cache
        volname    - set 8-character volume name
        !     - execute , then return
        quit
format> cache


CACHE MENU:
        write_cache - display or modify write cache settings
        read_cache  - display or modify read cache settings
        !      - execute , then return
        quit
cache> write      


WRITE_CACHE MENU:
        display     - display current setting of write cache
        enable      - enable write cache
        disable     - disable write cache
        !      - execute , then return
        quit
write_cache> display
Write Cache is disabled
write_cache> enable
write_cache> display
Write Cache is enabled
write_cache> q

Now, of course, be careful when you do this as data corruption can happen!

It was nice for me to do this as i was taking a simple v20z with a single SCSI disk to run some prelim specSFS numbers, and the write cache will mitigate (somewhat) the synchronous writes/commits. Of course you need much more than a single SCSI disk to get real specSFS numbers.

Or if you want to be snazier, Neil wrote a script to do this:

i_like_corruption# write_cache display c0t1d0
Write Cache is disabled
i_like_corruption# write_cache enable c0t1d0 
Write Cache is enabled
i_like_corruption# write_cache display c0t1d0
Write Cache is enabled
i_like_corruption# 

Here's the script:

#!/bin/ksh
#
# issue write cache commands to format(1M).
# commands can be to individual disks or all disks.
#
# usage: write_cache [-v] display|enable|disable all|
# E.g.:
#    write_cache disable all
#    write_cache -v enable c2t1d0

id=`id | sed -e 's/uid=//' -e 's/(.*//'`
if [ $id != "0" ] ; then
        printf "No permissions"
        exit 1
fi

# tmp files
cmds=/tmp/write_cache_commands.txt.$$
disks=/tmp/all_disk_list.txt.$$

silent=-s
if [ "$1" = "-v" ] ; then
        # in verbose mode turn off silent format option
        silent=
        shift
fi

cat > $cmds << EOF
cache
write_cache
$1
EOF

if [ "$2" = "all" ]; then
        echo disk | format 2>/dev/null | fgrep ". c" \
            | nawk '{ print $2 }' > $disks
        for i in `cat $disks`
        do
                format -e $silent -f $cmds $i 2>/dev/null
                if [ "$silent" = "-s" ]; then
                        # print write cache state using recursion
                        printf "%s : " $i
                        write_cache -v display $i | fgrep "Write Cache is"
                fi
        done
else
        format -e $silent -f $cmds $2
        if [ "$silent" = "-s" ]; then
                # print write cache state using recursion
                write_cache -v display $2 | fgrep "Write Cache is"
        fi
fi

rm -f $cmds $disks

Oh yeah, make sure its in your PATH. It can be run like this:

hodur# write_cache enable c4t2d0 
Write Cache is enabled
hodur# write_cache disable c4t2d0
Write Cache is disabled
hodur#