With only 1.3% read cache hits, RAID is reading 545MB/s to return 184MB/s to the client (200% backend overhead)

 System Performance Statistics
                All Ports     Port 1     Port 2     Port 3     Port 4
  Read  MB/s:      183.9       45.8       46.8       46.3       45.0
  Write MB/s:        0.0        0.0        0.0        0.0        0.0
  Total MB/s:      183.9       45.8       46.8       46.3       45.0

  Read  IO/s:        520        133        129        129        129
  Write IO/s:          0          0          0          0          0
  Total IO/s:        522        134        131        128        129

  Read Hits:         1.3%       1.6%       2.2%       0.6%       0.6%
  Prefetch Hits:     0.8%       1.1%       1.1%       0.6%       0.6%
  Prefetches:       46.3%      46.3%      46.0%      46.1%      46.8%
  Writebacks:        0.0%       0.0%       0.0%       0.0%       0.0%
  Rebuild MB/s:      0.0        0.0        0.0        0.0        0.0
  Verify MB/s:       0.0        0.0        0.0        0.0        0.0

                    Total      Reads     Writes      Pieces   Reads     Writes
  Disk IO/s:          518        518          0        1:      4910          0
  Disk MB/s:        544.6      544.6        0.0        2:     29890          0
  Disk Pieces:      65710      65710          0        3:       340          0
  BDB Pieces:                      0                   4:         0          0
                                                       5:         0          0
   Cache Writeback Data:     0.0%                      6:         0          0
   Rebuild/Verify Data:      0.0%    0.0%              7:         0          0
   Cache Data locked:        0.0%                      8:         0          0