Re: standard performance (write speed 20Mb/s)

Then you have multiple problems causing poor performance.  Now is the
time for you to post your  hardware configuration, drive model
number(s), mdadm config, and filesystem.  You should have done all of
this in your initial post.

hello and sorry for the late :-(

now is a resync in progress, I can't stop it. Try /usr/share/mdadm/checkarray 
-x md0 but nothing..

fs is ext3 (done less 10 days ago)

You've been on this list long enough to have known of the WD Green
sector alignment issue and avoided it, and to know better than to run
performance tests while an array is still in its initial sync.  Thus I
initially assumed your problem was merely the insane dd block size.

when I wrote a post there was not resync in progress..

tell me if you need known other things, thanks

Pol