Performance issues with Areca 1680 SAS Controllers

[Michael Fuckner <michael@fuckner.net>]

Hi,

I am using an Areca 1680-SAS-Controller with 16 SAS-HDD (Seagate 1TB
ST31000640SS). I set up a Raid6 with all 16 disks and formatted it with
XFS. The Controller has 512MB RAM and a BBU. The OS is installed to
another disk attached to the onboard AHCI controller.

After doing some IO, the areca raidset is slower compared to the rate 
directly
after boot.


[root@storage ~]# dd if=/dev/sdb1 of=/dev/null bs=1M count=50k iflag=direct
51200+0 records in
51200+0 records out
53687091200 bytes (54 GB) copied, 59.6494 seconds, 900 MB/s
[root@storage ~]# mount /dev/sdb1 /data
[root@storage ~]# cd /data/
[root@storage data]# ./iozone -i 0 -i 1 -s 32g -r 16m -S 6144 -t 8 -+r
-o  >raid6_sync_t8.log
[root@storage data]# cd
[root@storage ~]# umount /data/
[root@storage ~]# dd if=/dev/sdb1 of=/dev/null bs=1M count=50k iflag=direct
51200+0 records in
51200+0 records out
53687091200 bytes (54 GB) copied, 76.4036 seconds, 703 MB/s

I tested different Versions of Linux (Centos 5.2, OpenSUSE 11, Debian
Lenny) and Vanilla kernels 2.6.22-2.6.27, all show this behaviour.

Idea why the device slows down after IO- or better: how to keep the high
rate? Is this reproducible foer Areca SATA Controllers (Type 11XX and 12XX)

Regards,
   Michael!