strange performance of raid0

["Keld Jørn Simonsen" <keld@dkuug.dk>]

Hi

I have some strange performance results on a raid0
I have 4 IDE disks on two controllers, the one on the 
motherboard of the duron 1 GHz mackine, the other a promise TX2 plus 
SATA + PATA controller. I run kernel 2.4.22

The disks and hdparm -t on each of them

/dev/hdc1 seagate 80 GB                   16 MB/s
/dev/sda1 maxtor sata 200 GB              50 MB/s
/dev/sdb7 maxtor 160 GB                   54 MB/s

The partitions are al about 5 GB each.

If I make a raid0 device of all of them I get a thruput of 45 MB/s
IIf I exclude the hdc1 partition, I get around 75 MB/s.
The system is a little loaded - but that would be normal operating
conditions. CPU is 90 % idle. I have about 100 MB free RAM.

Why is it slower with 3 disks in the raid0 than
2 disks? Why don't I get approx 100 MB/s out of just the two?

Best regards
Keld