zfs: unexpected resilver of old disk when attaching a new mirror disk

[Martin Simmons]

When I attach a second disk to a single-disk zpool to make a mirrored pool,
the original disk is partially resilvered.  Is that expected?  It makes me
worry that some of the good data is being overwritten.

It happens on 8.0-RELEASE (pool version 13) and the 9.0-CURRENT-201004
snapshot (pool version 14).  It doesn't happen on Open Solaris snv_111b (also
pool version 14).

My setup commands for the test, making a 64MB zpool containing a 16MB file
are:

dd if=/dev/zero of=/tmp/zdisk0 bs=1m count=64
zpool create ztest /tmp/zdisk0
dd if=/dev/zero of=/ztest/a-file bs=1m count=16

The pool looks as expected:

  pool: ztest
 state: ONLINE
 scrub: none requested
config:

	NAME           STATE     READ WRITE CKSUM
	ztest          ONLINE       0     0     0
	  /tmp/zdisk0  ONLINE       0     0     0

errors: No known data errors

I then attach a second device:

dd if=/dev/zero of=/tmp/zdisk1 bs=1m count=64
zpool attach ztest /tmp/zdisk0 /tmp/zdisk1

and the pool looks like this after a while:

  pool: ztest
 state: ONLINE
 scrub: resilver completed after 0h0m with 0 errors on Thu Apr 22 00:29:32 2010
config:

	NAME             STATE     READ WRITE CKSUM
	ztest            ONLINE       0     0     0
	  mirror         ONLINE       0     0     0
	    /tmp/zdisk0  ONLINE       0     0     0  86K resilvered
	    /tmp/zdisk1  ONLINE       0     0     0  16.1M resilvered

errors: No known data errors

As expected, /tmp/zdisk1 was resilvered with 16MB of data for /ztest/a-file,
but why was a small amount of /tmp/zdisk0 also resilvered?

The amount varies varies (I've seen up to 220K) but it is never 0.  With real
disks constaining 5GB, the amount resilvered on the old disk is up to 70MB and
it increases gradually from 0 as the resilvering of the new disk proceeds.

__Martin