ZFS and idprio

[grarpamp]

On an i386 with 1GB ram and P4 2.x GHz single core with arc_max at
64MiB and everything else from RELENG_7_1_0_RELEASE left at default,
with this disk layering: PDC20269 at 100 > PATA > geli > zfs [gzip sha256].

The system become unusable slow when doing heavy sequential disk
IO. Such as copy 50G data, zpool scrub, etc. These two processes
[*] eat the cpu. I did idprio 31 on them and got 'ki-1' in the nice
column [was '-'] and seemed some responsiveness back after a few
minutes.

Should I be able to nice these kernel threads?
Any other way to drop their priority?
What is 'ki-1'? Top doesn't seem to show that field right as with
say nice -20 sleep 60?

Sorted by time:
   11 root        1 171 ki31     0K     8K RUN     25.2H 93.90% idle: cpu0
* 594 root        1  -8    -     0K     8K geli:w 321:27  0.68% g_eli[0] ad4
* 695 root        1 -16    -     0K    24K tq->tq 252:33  0.00% spa_zio_intr_1
* 597 root        1  -8    -     0K     8K geli:w 181:52  0.00% g_eli[0] ad5
* 729 root        1 101    -     0K    24K tq->tq 131:01  0.00% spa_zio_intr_1
  758 iso         1  44    0   172M 49572K select  62:43  0.00% XFree86
  740 root        1 -16    -     0K    24K tx->tx  44:00  0.00% txg_thread_enter

Not under load:
load averages: 0.12, 0.40, 0.51 up 2+01:11:50  14:19:22
164 processes: 4 running, 141 sleeping, 19 waiting
CPU: 0.8% user, 0.0% nice, 15.0% system, 0.8% interrupt, 83.5% idle
Mem: 147M Active, 146M Inact, 265M Wired, 24M Cache, 47M Buf, 410M Free