Linux Software RAID5 - ballpark numbers

Hello --

I realise this subject has been beaten to death, but I'd like to get a
handle on things. My basic system spec is in my sig. I'm running kernel
2.6.16.10.

I have recently built an additional array in the system, consisting of seven
250GB hard disks, configured for RAID 5 - total available storage is
1.46TB.

The array is split across two SIL3114A-based 4-channel PCI SATA controllers,
running in normal mode. One controller has 4 Hitachi 7K250 HDs, the other
has 2 Western Digital WD2500JD units and a single Seagate ST3250823AS.

Doing some basic tests in KDE 3.4.3, I copy a 4.4GB DVD image from an
external Maxtor OneTouch III 1TB FireWire 800 (internally this has two
500GB units configured RAID0). All filesystems are formatted with JFS.

Firstly, I done some individual tests on the drives, measuring the MB/s
unbuffered (sync option on the mount command) write speed. Results:

The four Hitachi 7K250s each averaged around 22.5MB/s write speed. The
Seagate averaged just over 23MB/s. The two Western Digital units each
averaged 30MB/s.

When I configured these as a RAID5 array, and did a copy of the same file
from the same source, the average write speed was 10Mb/s. Changing the
chunk size made absolutely no difference. The RAID 5 array was mounted
normally, i.e. buffered.

If I reconfigure the array as RAID0 (1.6TB capacity) I get an average
buffered write speed of around 12MB/s.

Now, I realise that having different disks from manufacturers doesn't help,
as there are clear performance discrepancies between the individual units,
but does that really explain such low performance? What numbers should I
really be seeing?

All help and advice gratefully received. Best regards,

Rohan Beckles
rohan.beckles@xxxxxxxxxx
--
ASUS PC-DL Deluxe
Intel Xeon @ 3.06GHz w/ 1MB L3 (dual)
Corsair TwinX-3200PT 2GB
Western Digital 2500SD (dual, RAID1)
eVGA nVidia 6800 Ultra 256MB

.