zdb -S でわかるらしいのでやってみた。


% zpool status -v
  pool: tank
 state: ONLINE
status: The pool is formatted using an older on-disk format.  The pool can
        still be used, but some features are unavailable.
action: Upgrade the pool using 'zpool upgrade'.  Once this is done, the
        pool will no longer be accessible on older software versions.
 scan: scrub repaired 0 in 6h35m with 0 errors on Thu May  5 10:04:39 2011
config:

        NAME         STATE     READ WRITE CKSUM
        tank         ONLINE       0     0     0
          raidz1-0   ONLINE       0     0     0
            ada1s2   ONLINE       0     0     0
            ada2s2   ONLINE       0     0     0
            ada3s2   ONLINE       0     0     0
        logs
          label/zil  ONLINE       0     0     0

errors: No known data errors
% zpool iostat
               capacity     operations    bandwidth
pool        alloc   free   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
tank        1.06T   898G     15     24   120K   299K
% zpool iostat -v
                capacity     operations    bandwidth
pool         alloc   free   read  write   read  write
-----------  -----  -----  -----  -----  -----  -----
tank         1.06T   898G     15     24   120K   299K
  raidz1     1.06T   898G     15     24   120K   251K
    ada1s2       -      -      4     11   163K   127K
    ada2s2       -      -      4     11   161K   127K
    ada3s2       -      -      4     11   161K   127K
  label/zil   204K  1.95G      0      0     14  48.5K
-----------  -----  -----  -----  -----  -----  -----

% zdb -S tank
Simulated DDT histogram:

bucket              allocated                       referenced
______   ______________________________   ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE   DSIZE
------   ------   -----   -----   -----   ------   -----   -----   -----
     1     336K   9.61G   9.61G   9.67G     336K   9.61G   9.61G   9.67G
     2    6.02K    215M    215M    216M    12.7K    442M    442M    444M
     4      592   9.52M   9.52M   9.64M    2.78K   43.3M   43.3M   43.9M
     8      223   1.77M   1.77M   1.83M    2.13K   18.2M   18.2M   18.7M
    16       19     39K     39K   42.6K      408    854K    854K    931K
    32        7     35K     35K   36.6K      315   1.96M   1.96M   2.03M
    64        5    130K    130K    131K      503   14.9M   14.9M   15.0M
   128        1     512     512     682      178     89K     89K    119K
   256        3   1.50K   1.50K   2.00K    1.17K    598K    598K    797K
   512        3   1.50K   1.50K   2.00K    2.16K   1.08M   1.08M   1.44M
    1K        1     512     512     682    1.20K    612K    612K    816K
 Total     343K   9.83G   9.83G   9.89G     359K   10.1G   10.1G   10.2G

dedup = 1.03, compress = 1.00, copies = 1.01, dedup * compress / copies = 1.02

%

表の意味はよくわからんが、最後の1行だけみると全く効果なしのようだ。