Stuttering I/O on DPT RAID

[Craig Boston]

Almost sent this to -current, I'm not used to 5.3 being stable yet...

I recently installed an old DPT RAID controller in a test machine (5.3-REL, 
SMP) and saw some odd I/O behavior.  The controller is:

dpt0: <DPT Caching SCSI RAID Controller> port 0xef80-0xef9f irq 17 at device 
16.0 on pci0
dpt0: DPT PM3334UW FW Rev. 07H1, 1 channel, 64 CCBs
dpt0: [GIANT-LOCKED]

da0 at dpt0 bus 0 target 0 lun 0
da0: <DPT RAID-5 07H1> Fixed Direct Access SCSI-2 device
da0: 17365MB (35564544 512 byte sectors: 255H 63S/T 2213C)

It's running RAID-5 and has an on-board cache (64MB worth of 72-pin SIMMs, 
heh).  The drives I think are Seagate Barracuda ultrawide, but I'm not 
physically there to verify at the moment -- they're in a locked enclosure.

The odd behavior surfaced when I went to zero out the array using dd with a 
1MB block size.  According to both gstat and iostat, the array is busy for 5 
seconds or so, then everything drops to 0 for about 2 seconds.

iostat -d -w 1 looks like this:

            da0
 KB/t tps  MB/s
128.00  14  1.73
128.00  20  2.48
128.00  20  2.48
128.00  19  2.35
128.00  19  2.35
 0.00   0  0.00
128.00   6  0.74
128.00  20  2.48
128.00  20  2.48
128.00  19  2.35
128.00  19  2.35
128.00   5  0.62
128.00   1  0.12

I know sequential writes aren't a very good way to measure performance, 
especially with RAID-5, but it just seemed a little... odd.  Is this to be 
expected?

Craig