Can not dump on raid dev?

[Kang Liu]

Hi,
	I'm trying to save a crashdump into /var/crash but it fails.
Here is my config files:
# dmesg | grep amr
amr0: <LSILogic MegaRAID> mem 0xf0000000-0xf7ffffff irq 3 at device 0.0 on pci3
amr0: <LSILogic PERC 3/DC> Firmware 1.92, BIOS 3.31, 128MB RAM
amrd0: <LSILogic MegaRAID logical drive> on amr0
amrd0: 17278MB (35385344 sectors) RAID 0 (optimal)
GEOM: create disk amrd0 dp=0xc7de088c
amrd1: <LSILogic MegaRAID logical drive> on amr0
amrd1: 51834MB (106156032 sectors) RAID 0 (optimal)
GEOM: create disk amrd1 dp=0xc7de078c
ses0 at amr0 bus 0 target 6 lun 0
Mounting root from ufs:/dev/amrd0s1a

in rc.conf:
dumpdev="/dev/amrd0s1b"  
dumpdir="/var/crash"

in my kernel conf-file:
options         DDB
option          DDB_UNATTENDED
makeoptions     DEBUG=-g

# disklabel -r /dev/amrd0
# /dev/amrd0:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a:   524288        0    4.2BSD     2048 16384 32776 
  b:  8345024   524288      swap                    
  c: 35385344        0    unused        0     0         # "raw" part, don't edit
  d:   524288  8869312    4.2BSD     2048 16384 32776 
  e:   524288  9393600    4.2BSD     2048 16384 32776 
  f: 25467456  9917888    4.2BSD     2048 16384 28552 

# cat /etc/fstab |grep amrd0s1b
/dev/amrd0s1b           none            dump    sw              0       0

#before I start the test, I run "dumpon -v /dev/amrd0s1b". It seems ok.
# dumpon -v /dev/amrd0s1b
kernel dumps on /dev/amrd0s1b

when the system crashes, it doesn't save any crashdump. 
I've tried the same config on a machine with IDE disk, the crashdump can be saved.
Do I miss something, or the crashdump can not be saved on a raid device? 


Kang.