zpool - low speed write

[Alex V. Petrov]

Hi All!

$ dd if=/dev/random of=/tank/test bs=3M count=1000
1000+0 records in
1000+0 records out
3145728000 bytes transferred in 298.153293 secs (10550707 bytes/sec)

What, i think, very-very low :-(

FreeBSD alex.super 8.1-STABLE FreeBSD 8.1-STABLE #76: Mon Aug  2 20:19:09 
KRAST 2010     alex at alex.super:/usr/obj/usr/src/sys/ALEX  amd64

real memory  = 4294967296 (4096 MB)
avail memory = 4098732032 (3908 MB)

zpool iostat -v
               capacity     operations    bandwidth
pool         used  avail   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
tank        1,91T   829G    101      4  12,4M   148K
  ada4       597G   334G     31      1  3,81M  53,4K
  ada2       633G   295G     32      1  4,03M  51,8K
  ada3       727G   201G     37      1  4,59M  42,9K
----------  -----  -----  -----  -----  -----  -----

zpool status -v
  pool: tank
 state: ONLINE
 scrub: scrub completed after 7h12m with 0 errors on Tue Aug  3 04:54:14 2010
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          ada4      ONLINE       0     0     0
          ada2      ONLINE       0     0     0
          ada3      ONLINE       0     0     0

errors: No known data errors

zpool history
History for 'tank':
2009-07-16.19:46:24 zpool create tank ad12
2009-12-13.14:58:46 zpool add tank ad8
2010-04-24.01:59:41 zpool upgrade tank
2010-05-09.02:16:34 zpool add tank ada3
2010-05-25.17:57:12 zpool scrub tank
2010-06-27.16:02:45 zpool scrub tank
2010-08-02.21:41:53 zpool scrub tank

ada2 at ahcich2 bus 0 scbus2 target 0 lun 0
ada2: <WDC WD10EADS-00M2B0 01.00A01> ATA-8 SATA 2.x device
ada2: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada2: Command Queueing enabled
ada2: 953869MB (1953525168 512 byte sectors: 16H 63S/T 16383C)
ada3 at ahcich3 bus 0 scbus3 target 0 lun 0
ada3: <WDC WD10EADS-00L5B1 01.01A01> ATA-8 SATA 2.x device
ada3: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada3: Command Queueing enabled
ada3: 953869MB (1953525168 512 byte sectors: 16H 63S/T 16383C)
ada4 at ahcich4 bus 0 scbus4 target 0 lun 0
ada4: <WDC WD10EADS-00M2B0 01.00A01> ATA-8 SATA 2.x device
ada4: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada4: Command Queueing enabled

smartd daily output:
Checking health of /dev/ada2: OK
Checking health of /dev/ada3: OK
Checking health of /dev/ada4: OK

/boot/loader.conf:
ahci_load="YES"
sem_load="YES"
snd_hda_load="YES"
nvidia_load="YES"
linux_load="YES"
wlan_xauth_load="YES"
vboxdrv_load="YES"
atapicam_load="YES"
coretemp_load="YES"
aio_load="YES"
vfs.zfs.prefetch_disable=0
vfs.zfs.cache_flush_disable=1
vfs.zfs.zil_disable=1

Any ideas?
-----
Alex V. Petrov