Linux md vs. LVM performance

I am trying to tune my NAS, running openfiler, and wondering why I'm getting relatively poor read performance from 4 WD RE3 drives in RAID 5.

EDIT: Please note I am talking about the buffered disk read speed not cached speeds

EDIT: Changed formatting to make clear there are two sets of output.

When I run hdparm on the meta device I get the levels of performance I'd expect, drop to the volume and it's a third the speed !

Any one any idea why ? Is LVM that bad ?

Dean

Meta device /dev/md0 results

[root@nas2 etc]# hdparm -tT /dev/md0
/dev/md0:
 Timing cached reads:   4636 MB in  2.00 seconds = 2318.96 MB/sec
 Timing buffered disk reads:  524 MB in  3.01 seconds = 174.04 MB/sec

Vol group /dev/mapper/vg1-vol1 results

[root@nas2 etc]# hdparm -tT /dev/mapper/vg1-vol1
/dev/mapper/vg1-vol1:
 Timing cached reads:   4640 MB in  2.00 seconds = 2320.28 MB/sec
 Timing buffered disk reads:  200 MB in  3.01 seconds =  66.43 MB/sec

Edit: See section from the hdparm man page which suggest this is perfectly valid test for sequential read performance which is the issue I am trying to resolve.

 -t     Perform timings of device reads for benchmark and comparison purposes.  For meaningful results, this operation should be repeated 2-3 times on an otherwise
              inactive system (no other active processes) with at least a couple of megabytes of free memory.  This displays the speed  of  reading  through  the  buffer
              cache  to  the  disk  without  any  prior caching of data.  This measurement is an indication of how fast the drive can sustain sequential data reads under
              Linux, without any filesystem overhead.  To ensure accurate measurements, the buffer cache is flushed during the  processing  of  -t  using  the  BLKFLSBUF
              ioctl.   If  the  -T  flag  is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t
              operation.