I've some important information that should shed some light on this behavior:

This evening I created a new filesystem across the very same 50 disks including 
the COMPRESS attribute. My goal was to isolate some workload to the new 
filesystem and started moving a 100GB directory tree over to the new FS. While 
I was copying I was averaging around 25MB read and 25MB write as expected. 
[b]Now I opened 'vi' and wanted to write out a new file in the new filesystem 
and what I saw was shocking: my reads remained the same but my writes shot upto 
the 150+MB/S range. This abnormal I/O pattern continued until the 'vi' returned 
from the write request.[/b] Here are the 'zpool iostat mtdc 30' output:

               capacity     operations    bandwidth
pool         used  avail   read  write   read  write
----------  -----  -----  -----  -----  -----  -----
mtdc         806G  2.48T     38    173  1.93M  7.52M
mtdc         806G  2.48T    188    228  15.0M  8.78M
mtdc         807G  2.48T    266    624  14.0M  16.5M
mtdc         807G  2.48T    286    670  17.1M  14.5M
mtdc         807G  2.48T    293  1.21K  18.2M  98.4M <<-- vi activity, note 
mismatch in r/w rates
mtdc         808G  2.48T    457    560  35.5M  24.2M
mtdc         809G  2.48T    405    504  31.7M  26.3M
mtdc         809G  2.48T    328  1.37K  25.2M   152M <<-- vi activity, note r/w 
mismatch in r/w rates
mtdc         810G  2.48T    428    671  33.0M  48.0M
mtdc         811G  2.48T    463    500  35.9M  26.4M
mtdc         811G  2.48T    207  1.39K  16.5M   154M<<-- vi activity, note r/w 
mismatch in r/w rates
mtdc         812G  2.48T    310    878  23.9M  77.7M
mtdc         813G  2.48T    362    494  26.1M  25.3M
mtdc         813G  2.48T    381  1.05K  26.8M   103M
mtdc         814G  2.48T    347  1.33K  25.0M   135M
mtdc         815G  2.48T    288  1.38K  21.7M   150M
mtdc         815G  2.48T    425    513  32.7M  25.8M
mtdc         816G  2.47T    413    515  30.2M  25.1M
mtdc         817G  2.47T    341    512  21.9M  25.1M
mtdc         818G  2.47T    293    529  18.5M  25.5M
mtdc         818G  2.47T    344    508  23.4M  24.7M
mtdc         819G  2.47T    442    512  33.4M  24.1M
mtdc         820G  2.47T    385    483  28.3M  24.4M
mtdc         820G  2.47T    372    483  24.7M  24.7M
mtdc         821G  2.47T    347    535  23.0M  24.2M
mtdc         821G  2.47T    290    497  17.9M  24.9M
mtdc         823G  2.47T    349    517  20.0M  24.1M
mtdc         823G  2.47T    399    512  21.2M  24.5M
mtdc         824G  2.47T    383    612  19.3M  17.7M
mtdc         824G  2.47T    390    614  14.2M  17.5M
 
 
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