userland falling over from pata writes on a 4.x box

[Daniel Hagerty <hag%linnaean.org@localhost>]

    I have an old p2-450 box on an p2b-s motherboard running the head
of netbsd-4 from last march or so.  I just saw it's userland fall over
while doing heavy writes to one of its PATA disk.  For example,
getting an ssh banner string via telnet from another machine measured
in minutes.  This was part of a broader bit shuffle, and it was only
writes to the pata disk that were so obviously pathological.

    This is not my memory of how it used to behave, but it's been a
long time since the machine has been regularly used for I/O intensive
work.

    The salient bits from dmesg are appended to this message.

    I'm particularly concerned that it's reporting the piixide as
wired to compatability mode, which is not what I remember from the
distant past.  I can't find anything in its bios mentioning legacy ide
vs pci ide, merely an enable both/primary/secondary/neither and knobs
for which dma + pio modes to use.

    Short of upgrading the machine, any ideas what to look at?

NetBSD 4.0_STABLE (PERDITION) #0: Mon Mar  8 09:49:52 EST 2010
[...]
piixide0 at pci0 dev 4 function 1
piixide0: Intel 82371AB IDE controller (PIIX4) (rev. 0x01)
piixide0: bus-master DMA support present
piixide0: primary channel wired to compatibility mode
piixide0: primary channel interrupting at irq 14
atabus0 at piixide0 channel 0
piixide0: secondary channel wired to compatibility mode
piixide0: secondary channel interrupting at irq 15
atabus1 at piixide0 channel 1
[...]
satalink0 at pci0 dev 9 function 0
satalink0: Silicon Image SATALink 3112 (rev. 0x02)
satalink0: SATALink BA5 register space disabled
satalink0: bus-master DMA support present
satalink0: primary channel wired to native-PCI mode
satalink0: using irq 5 for native-PCI interrupt
atabus2 at satalink0 channel 0
satalink0: secondary channel wired to native-PCI mode
atabus3 at satalink0 channel 1
[...]
satalink0: port 0: device present, speed: 1.5Gb/s
satalink0: port 1: device present, speed: 1.5Gb/s
atapibus0 at atabus0: 2 targets
cd0 at atapibus0 drive 1: <MAD DOG MD-16XDVD9A4, , 1.F0> cdrom removable
cd0: 32-bit data port
cd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2 (Ultra/33)
wd0 at atabus0 drive 0: <ST360021A>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 57241 MB, 116301 cyl, 16 head, 63 sec, 512 bytes/sect x 117231408 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd0(piixide0:0:0): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA)
cd0(piixide0:0:1): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA)
wd1 at atabus1 drive 0: <Maxtor 6L100P0>
wd1: drive supports 16-sector PIO transfers, LBA addressing
wd1: 95611 MB, 194259 cyl, 16 head, 63 sec, 512 bytes/sect x 195813072 sectors
wd1: 32-bit data port
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd1(piixide0:1:0): using PIO mode 4, Ultra-DMA mode 2 (Ultra/33) (using DMA)
wd2 at atabus2 drive 0: <ST3808110AS>
wd2: quirks 2<FORCE_LBA48>
wd2: drive supports 16-sector PIO transfers, LBA48 addressing
wd2: 76319 MB, 155061 cyl, 16 head, 63 sec, 512 bytes/sect x 156301488 sectors
wd2: 32-bit data port
wd2: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd2(satalink0:0:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
wd3 at atabus3 drive 0: <ST3808110AS>
wd3: quirks 2<FORCE_LBA48>
wd3: drive supports 16-sector PIO transfers, LBA48 addressing
wd3: 76319 MB, 155061 cyl, 16 head, 63 sec, 512 bytes/sect x 156301488 sectors
wd3: 32-bit data port
wd3: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd3(satalink0:1:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)