ZFS v28 array doesn't expand with larger disks in mirror

[Kurt Touet]

I have an admittedly odd zfs v28 array configuration under stable/8 r223484:

# zpool status storage
  pool: storage
 state: ONLINE
 scan: resilvered 1.21T in 10h50m with 0 errors on Wed Jun 29 23:21:46 2011
config:

        NAME        STATE     READ WRITE CKSUM
        storage     ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            ad14    ONLINE       0     0     0
            ad6     ONLINE       0     0     0
            ad12    ONLINE       0     0     0
            ad4     ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            ad20    ONLINE       0     0     0
            ad18    ONLINE       0     0     0

This was simply due to the need to expand the size of the original
raidz1 only array and constraints within the box.  All drives in the
box _were_ 1.5TB.  I had a drive in the mirror die this week, and I
had 2 spare 2TB drives on hand.  So, I decided to replace both of the
1.5TB drives in the array with 2TB drives (and free up a little more
space on the box).  However, after replacing both drives, the array
did not expand in size.  It still acts as if the mirror contains 1.5TB
drives:

storage     6.28T   548G
  raidz1    5.07T   399G
  mirror    1.21T   150G

Is this normal behaviour?  It was my understanding that zfs
automatically adapted to having additional drive space in vdevs.

-kurt