I recall trying to find this information quite a while ago. My memory may be failing me, but what I recall is that you can use fstat to find the processes that are writing to your disks (and the files they're writing to) for most filesystems. Except ZFS. If it's ZFS, then you have to use some convoluted debugging method to find this information. In my case, ZFS was the culprit, and it turned out to be such a pain in the behind that I gave up and stopped worrying about it.
Hopefully the experts will chime in to correct my errors.
EDIT: Looking over the output of fstat on my box, it seems like maybe you get the process and the filesystem, but not the exact file path for ZFS. Maybe finding the exact path is what I'm remembering being a pain.
