Here I announce 2nd version my new barrier support patches: http://people.redhat.com/mpatocka/patches/kernel/barriers/ They are against 2.6.29-rc3.The most important enhancement is that with these patches, device mapper is able to send barrier requests to lower devices.
I guess it still won't work for those using LVM on md (Linux software RAID) because md lacks barrier support? Do you know if someone is working on it?
-- Tomasz Chmielewski http://wpkg.org