RAID5 full stripe writes

Hello,

I created a fresh VxVM 6.0 RAID5 volume on four columns. I performed
some basic I/O operations and noticed that writes always end in
read-modify-write cycle. I haven't been able to trigger a full stripe
writes yet, regardless of the command I used to write to the
filesystem. I tried with 64k and with 128k stripe width but in both
cases, I cannot get writes aligned. Filesystem is VxFS.
Below are outputs of my simple tests and configuration of the RAID5
volume. Any hints where I got it wrong?

1. dd example
root <at> mlincek:~# dd if=/dev/zero of=/mlincek/zero.dd bs=384k

root <at> mlincek:~# iostat -zxn 2
                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
   69.0  115.4 8827.8 14777.0  0.0  0.4    0.0    2.3   0  41 c0t2d0
   69.0  115.4 8827.8 14777.0  0.0  0.3    0.0    1.4   0  25 c0t3d0
   70.0  115.4 8955.8 14777.0  0.0  0.4    0.0    2.0   0  36 c0t5d0
   69.5  115.9 8891.8 14841.0  0.0  0.4    0.0    2.2   0  41 c0t4d0

2. Copy file from another physical volume using cp command:

root <at> mlincek:~# cp testfile.tmp /mlincek/

                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
   19.0  159.9 2250.5 20290.3  0.0  0.5    0.0    2.7   0  40 c0t2d0
   17.5  158.4 2239.0 20278.9  0.0  0.3    0.0    1.5   0  16 c0t3d0