ZFS prefetch efficiency

[Anton Sayetsky]

Hello,

I have a machine with 256 G RAM almost all dedicated to ARC. zfs-stats
shows the following:
root at cs0:~# zfs-stats -EZ

------------------------------------------------------------------------
ZFS Subsystem Report                            Tue Apr 15 15:25:10 2014
------------------------------------------------------------------------

ARC Efficiency:                                 1.07b
        Cache Hit Ratio:                73.53%  788.04m
        Cache Miss Ratio:               26.47%  283.62m
        Actual Hit Ratio:               64.30%  689.08m

        Data Demand Efficiency:         99.69%  359.48m
        Data Prefetch Efficiency:       31.54%  409.48m

        CACHE HITS BY CACHE LIST:
          Anonymously Used:             10.38%  81.78m
          Most Recently Used:           38.09%  300.13m
          Most Frequently Used:         49.36%  388.95m
          Most Recently Used Ghost:     0.56%   4.45m
          Most Frequently Used Ghost:   1.62%   12.74m

        CACHE HITS BY DATA TYPE:
          Demand Data:                  45.48%  358.38m
          Prefetch Data:                16.39%  129.15m
          Demand Metadata:              37.79%  297.81m
          Prefetch Metadata:            0.34%   2.69m

        CACHE MISSES BY DATA TYPE:
          Demand Data:                  0.39%   1.10m
          Prefetch Data:                98.84%  280.33m
          Demand Metadata:              0.76%   2.16m
          Prefetch Metadata:            0.01%   39.25k

------------------------------------------------------------------------

File-Level Prefetch: (HEALTHY)

DMU Efficiency:                                 3.30b
        Hit Ratio:                      91.98%  3.04b
        Miss Ratio:                     8.02%   264.94m

        Colinear:                               264.94m
          Hit Ratio:                    0.01%   20.57k
          Miss Ratio:                   99.99%  264.92m

        Stride:                                 2.77b
          Hit Ratio:                    99.99%  2.77b
          Miss Ratio:                   0.01%   245.81k

DMU Misc:
        Reclaim:                                264.92m
          Successes:                    0.60%   1.60m
          Failures:                     99.40%  263.32m

        Streams:                                270.09m
          +Resets:                      0.06%   164.49k
          -Resets:                      99.94%  269.93m
          Bogus:                                0

------------------------------------------------------------------------

root at cs0:~#

I'm confused with the next 2 values:
1.         Data Prefetch Efficiency:       31.54%  409.48m
2. DMU Efficiency:                                 3.30b
        Hit Ratio:                      91.98%  3.04b

So here is my question: is prefetch really efficient or not?