[ubuntu] mdadm raid 1 slow

hi all,

I have build myself a small nas as test.
It is using ubuntu server 12.10

The installation is done on a 4 GB usb stick (this I know is slow for boot)
2 hard-disks of 1 TB are being used as raid 1

When I test the local

Here are some settings:

sudo mdadm --detail /dev/md0
[sudo] password for wim:
/dev/md0:
Version : 1.2
Creation Time : Tue Nov 27 17:00:51 2012
Raid Level : raid1
Array Size : 976631360 (931.39 GiB 1000.07 GB)
Used Dev Size : 976631360 (931.39 GiB 1000.07 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent

Update Time : Mon Dec 3 10:34:57 2012
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Name : nas:0 (local to host nas)
UUID : ebfa17a9:915551f4:e45384b8:59877a13
Events : 19

Number Major Minor RaidDevice State
0 8 0 0 active sync /dev/sda
1 8 16 1 active sync /dev/sdb

Hdparm individual drives:

sudo hdparm -tT /dev/sda

/dev/sda:
Timing cached reads: 8742 MB in 2.00 seconds = 4374.50 MB/sec
Timing buffered disk reads: 516 MB in 3.00 seconds = 171.88 MB/sec

/dev/sdb:
Timing cached reads: 8598 MB in 2.00 seconds = 4302.40 MB/sec
Timing buffered disk reads: 396 MB in 3.01 seconds = 131.59 MB/sec

Hdparm for the raid set:

/dev/md0:
Timing cached reads: 8538 MB in 2.00 seconds = 4272.31 MB/sec
Timing buffered disk reads: 508 MB in 3.00 seconds = 169.14 MB/sec

Signaling of the sata interface

wim@nas:~$ sudo hdparm -I /dev/sda | grep -i speed
* Gen1 signaling speed (1.5Gb/s)
* Gen2 signaling speed (3.0Gb/s)
* Gen3 signaling speed (6.0Gb/s)

wim@nas:~$ sudo hdparm -I /dev/sdb | grep -i speed
* Gen1 signaling speed (1.5Gb/s)
* Gen2 signaling speed (3.0Gb/s)

Now, why is writing so slow to this setup?

When I write using samba share I get around 25Mb/s = around 180Mbps
The network is full gigabit and on my real server (other not ubuntu) I get writing speeds of around 98Mb/s

When I test in linux it shows 340 MB/s

wim@nas:/mnt/DATA$ dd if=/dev/zero of=/mnt/DATA/out.img bs=8k count=256k
262144+0 records in
262144+0 records out
2147483648 bytes (2.1 GB) copied, 6.31609 s, 340 MB/s

SO, I think theoretical I can get 340 MB/s = fast enough this is about gigabit speed
so loosing half speed is what I should want to achief. around 100MB/s is enough.

But I only get 23. What can I see to adjust to get it better.

Ubuntu gots 8Gb of ram and 4core AMD FX 4100 3GHz CPU
Should all be fast enough.

Suggestions???
thanks.

FS= EXT4