quoted

        16k read        64k write
chunk
size    RAID 5  RAID 6  RAID 5  RAID 6
128k    492     497     268     270
256k    615     530     288     270
512k    625     607     230     174
1024k   650     620     170     75

It strikes me that these numbers are meaningless without knowing if
that is actual data-to-disk or data-to-memcache-and-some-to-disk-too.
Later versions of 'dd' offer 'conv=fdatasync' which is really handy
(call fdatasync on the output file, syncing JUST the one file, right
before close). Otherwise, oflags=direct will (try) to bypass the
page/block cache.

I can get really impressive numbers, too (over 200MB/s on a single
disk capable of 70MB/s) when I (mis)use dd without fdatasync, et al.

The variation in reported performance can be really huge without
understanding that you aren't actually testing the DISK I/O but *some*
disk I/O and *some* memory caching.

I did these benchmarks with 32GB of data on a machine with 1GB of RAM, 
therefore the memory cache contribution should be small.

Kind regards,

Thiemo