ata timeouts under load

I am getting timeouts on 8.0b4/HEAD when I do a lot of ZFS I/O to a pool 
on ad4:

atapci0: <VIA 6420 SATA150 controller> port 
0xc800-0xc807,0xc400-0xc403,0xc000-0xc007,0xb800-0xb803,0xb400-0xb40f,0xb000-0xb0ff 
irq 20 at device 15.0 on pci0
ata2: <ATA channel 0> on atapci0
ata3: <ATA channel 1> on atapci0
ata0: <ATA channel 0> on atapci1
ata1: <ATA channel 1> on atapci1

ad4: 476940MB <WDC WD5000AAKS-00TMA0 12.01C01> at ata2-master SATA150
ad4: WARNING - SETFEATURES SET TRANSFER MODE taskqueue timeout - 
completing request directly
ad4: WARNING - SETFEATURES SET TRANSFER MODE taskqueue timeout - 
completing request directly
ad4: WARNING - SETFEATURES ENABLE RCACHE taskqueue timeout - completing 
request directly
ad4: WARNING - SETFEATURES ENABLE WCACHE taskqueue timeout - completing 
request directly
ad4: WARNING - SET_MULTI taskqueue timeout - completing request directly
ad4: TIMEOUT - WRITE_DMA48 retrying (1 retry left) LBA=344052040
ad4: WARNING - SETFEATURES SET TRANSFER MODE taskqueue timeout - 
completing request directly
ad4: WARNING - SETFEATURES SET TRANSFER MODE taskqueue timeout - 
completing request directly

It becomes stuck in a loop displaying the above and is unable to 
complete further I/O operations.  I wonder if it is just batching up a 
lot of I/O and then timing out because it is busy, and then not 
recovering from this state?

Any ideas what could be wrong?

Kris