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

[Adam Nowacki]

My best guess is metadata updates to refer to the 2nd disk. I've seen 
same behavior when replacing a bad disk in 10 disk raidz2 pool. No data 
was affected.

Martin Simmons wrote:
> 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
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