Postgresql in Xen DomU

Hello List,

i have a few postgresql servers running in a Xen3/DomU and i am having
real bead performance...

Since read-ahead seems to be a big tuning detail, i would like to know
where i have to set my read-ahead size?
Do i have to set it in the Dom0 or DomU?

My layers looks like this:

1.) Hard Disk (Dom0)
2.) LVM Partitions (Dom0)
3.) sda disk with ext3 (DomU)

Or does someone have some experience with Xen and Postgresql?

Thanks,
Mario