raidz2 a bit big

[Randy Bush]

a dozen 2tb drives in a raidz2

dfw1.psg.com:/root# zpool status
  pool: tank
 state: ONLINE
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          raidz2    ONLINE       0     0     0
            da0s3   ONLINE       0     0     0
            da1s3   ONLINE       0     0     0
            da2s1   ONLINE       0     0     0
            da3s1   ONLINE       0     0     0
            da4s1   ONLINE       0     0     0
            da5s1   ONLINE       0     0     0
            da6s1   ONLINE       0     0     0
            da7s1   ONLINE       0     0     0
            da8s1   ONLINE       0     0     0
            da9s1   ONLINE       0     0     0
            da10s1  ONLINE       0     0     0
            da11s1  ONLINE       0     0     0

errors: No known data errors

i expected to get 10T effective because of the raidz2, but got 20T
                                                    ^
dfw1.psg.com:/root# df -H
Filesystem        Size    Used   Avail Capacity  Mounted on
/dev/da0s1a        17G    3.8G     12G    25%    /
devfs             1.0k    1.0k      0B   100%    /dev
/dev/md0          130M     90k    119M     0%    /tmp
tank               19T      0B     19T     0%    /tank
tank/usr           19T      0B     19T     0%    /tank/usr
tank/usr/home      19T      0B     19T     0%    /tank/usr/home
tank/var           19T      0B     19T     0%    /tank/var
tank/var/spool     19T      0B     19T     0%    /tank/var/spool
tank/data          19T      0B     19T     0%    /tank/data

clue bat?

randy