completely stopped work on the ROOT zfs

I read
http://wiki.freebsd.org/ZFSTuningGuide
http://wiki.freebsd.org/ZFSKnownProblems

set up with defferent amount of memory in a month.
file system completely stops its work, even if not very
large loads. do not panic the kernel and core file.

uname -a
FreeBSD ss.su 8.0-CURRENT FreeBSD 8.0-CURRENT #1 r187593: Fri Jan 23 
22:58:42 KRAT 2009     zloiadmin_at_ss.su:/usr/obj/usr/src/sys/zl0_J16  i386

option in my kernel

options         KVA_PAGES=512

real memory  = 2112749568 (2014 
MB)                                                                                         

avail memory = 2056982528 (1961 MB)

ad0: 114473MB <WDC WD1200BEVE-00UYT0 01.04A01> at ata0-master 
UDMA100                                                      
ZFS filesystem version 
13                                                                                                   

ZFS storage pool version 
13                                                                                                 

acd0: DVDR <NEC DVD+/-RW ND-6450A/2.36> at ata1-master 
PIO4                                                                
WARNING: WITNESS option enabled, expect reduced 
performance.                                                               
Trying to mount root from zfs:h 

loader.conf
zfs_load=YES
vfs.root.mountfrom="zfs:h"
vm.kmem_size="1536M"  modify the values of 512 to 1536
vm.kmem_size_max="1536M" and here
vfs.zfs.arc_max="150M"     of 50 to 700
vfs.zfs.vdev.cache.size="5M"   of 1 to 50
vfs.zfs.prefetch_disable="1"
vfs.zfs.zil_disable="1"

i use script on http://wiki.freebsd.org/ZFSTuningGuide
TEXT=12481192, 11,903 
MB                                                                                                    

DATA=634163200, 604,785 
MB                                                                                                  

TOTAL=646644392, 616,688 MB

how to get the core-file, or to solve this problem?