zfs unneceary massive IO writing

Thu Nov 28 21:57:31 UTC 2013

I have host with 8-STABLE and ZFS.
ZFS setup is very simple:

zfs_www      880G   665G   215G    75%  1.00x  ONLINE  -  
  aacd0s3    880G   665G   215G         -

No L2ARC, no ZIL, copies=1

After creating small file (or append some bytes to file) I see very
large write to disk (400 write ops, total up to 1MB).
How I can discover this problem?
dtrace on zio_create? How I can diagnose source of IO operation?