Thanks for your help
here is the disk connection map
Host bridge: Intel Corporation Xeon E7 v2/Xeon E5 v2/Core i7 DMI2 (rev 04)
PCI bridge: Intel Corporation Xeon E7 v2/Xeon E5 v2/Core i7 PCI Express Root Port 2c (rev 04)
RAID bus controller: Broadcom / LSI SAS2008 PCI-Express Fusion-MPT SAS-2 [Falcon] (rev 03)
13 drives(left 13 in the picture ,main disks in parity sync situation)
PCI bridge: Intel Corporation Xeon E7 v2/Xeon E5 v2/Core i7 PCI Express Root Port 3a (rev 04)
Serial Attached SCSI controller: Broadcom / LSI SAS2008 PCI-Express Fusion-MPT SAS-2 [Falcon] (rev 03)
34 drives
and it is going down to 14m/s ……
I cant figure out why this is happening ,all the 8 disk(2parity and 6 data disks) are in the first bus, and no background app is running .