zfs code - Dec 2008 - interchangeable vdev - mystery bytes in vdev label

So I have a kludgy way of pinning writes to one vdev (see my other post -
http://www.opensolaris.org/jive/thread.jspa?threadID=85261&tstart=0).  This
doesn''t quite get the read-only vdev I wanted (writes are still
occurring in the vdev label).  My ultimate goal is to have an interchangeable
vdev which will serve as the base of all my filesystems.

In my investigation to complete an interchangeable vdev - I''ve noticed
48 mystery bytes being written at the end of the nv-pair section of the vdev
label.  From 0x0001 FFD0 to 0x0002 0000 on a vdev, there are 48 bytes of data
that gets changed on import.  The on-disk specification states that 16K - 128K
is dedicated to name value pairs (0x0000 4000 to 0x0002 0000).  I''m
going to start tracing through the import source now w/ a debugger, but
I''m not sure how easy it is going to be to see when this section is
written (for my own sake, I hope it''s grossly obvious but things
usually don''t work out that way...).

If you know what these 48 bytes might be - please let me know!  I''ll
post what I find.

~Mike
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