very slow zfs scrub

[Piotr Kucharski]

I've eliminated GELI from the picture and simplified the setup to one
ggate disk.

# ggatec create -u 13 x.x.x.x /dev/zvol/storage/zvol/temp
# zpool create testink /dev/ggate13
: copied some files onto it, 10G took 40 min, but it was from one
network drive to another, I'll let it slide
# dd if=/testink/file of=/dev/null bs=64k
128712704 bytes transferred in 2.256507 secs (57040687 bytes/sec)
: reading is as I expected to be, fine
# zpool scrub testink
: wait a little for the command to finish
# zpool status testink
  pool: testink
 state: ONLINE
 scan: scrub in progress since Thu Feb 24 17:45:27 2011
    227M scanned out of 9.97G at 27.9K/s, 101h52m to go
    0 repaired, 2.22% done
config:

        NAME        STATE     READ WRITE CKSUM
        testink     ONLINE       0     0     0
          ggate13   ONLINE       0     0     0

Wow! What does scrub do that it slows ggate drive almost to halt?

What can I do to fix it?