3x read to write ratio on dump/restore

[M. Warner Losh]

I just copied a disk using dump + restore.  I noticed something
through the whole run of this 500GB operation:

 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
    3    231    231  14754    9.5      0      0    0.0   97.5| da1s1a
    0     39      0      0    0.0     39   4982   10.0   38.7| da2s1a

The read kBps was 3x the write kBps.  While the dump is going through
the raw device, and the restore is going through the file system, I
can't imagine why we'd have such a huge difference that would be utter
consistent for the whole 15 hour run.

Any ideas what gives?  I observed this with 16MB cache and with 32MB
cache, fwiw.

Warner