LVM over MD performance

Hi,

I have a setup with 15 SATA drives configured as 3 x SW RAID with 5 drives each with 64k stripe size. Configuration: kernel version is 2.6.18-8.1.3.el5, CPU Xeon 5130, LSI PCI-X SAS controller. I am testing the devices in parallel with seq I/O using Oracle Orion and 1 MB block size.

3 x MD RAID 5 (all values in MB/s):

Read:  800.42
Write: 564.75

When I setup a volume group + lv on each of the three devices I get the following results:

Read:  729.28 (-9 %)
Write: 362.30 (- 36 %)

Is there a reason that write performance is so much slower?
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Striping: I would like to aggregate the individual RAID into a single vg with a lv using all disks. Using md with RAID 0 over 3 x RAID 5 I get the following results (stripe size is 256k):

Read:  700.68
Write: 530.43

When using LVM to do the striping (-i 3 -I 256K) I get the following:

Read:  660.71 (- 6%)
Write: 149.29 (- 72%)

Should one avoid LVM striping or are there some parameters that need to be adjusted?

Regards,
Mirko