Garbled console output on Pi3

[bob prohaska]

The j8 buildworld ended with a crash. Console, backtrace, gstat log, buildworld
log and top window are available at
http://www.zefox.net/~fbsd/rpi3/crashes/20180307/
in case they're of interest. 

The most curious feature to my untrained eye is the gstat log, which ends with

dT: 10.002s  w: 10.000s
 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w    d/s   kBps   ms/d   %busy Name
    3      2      0     10  27226      1     58  27002      0      0    0.0  275.3  da0
    1      0      0      3   36.7      0      0    0.0      0      0    0.0    0.4  mmcsd0s3
    0      0      0      0    0.0      0      6  26958      0      0    0.0  275.3  da0a
    3      1      0     10  27246      1     52  26875      0      0    0.0  275.6  da0d

On this host da0a is /var and da0d is /usr. There are 3GB of swap on 
mmcsd0s3b and d, which don't seem particularly busy. Nonetheless,
/var and /usr are showing a physically impossible level of activity.

I hope this makes sense to somebody.....

bob prohaska