Originally Posted by gattler
The last command, the most important one, wasn't explained enough, could someone please elaborate how to extract the free space from the Volume Group and "map" it to a Physical Volume?
vgreduce vg_blah /dev/sdxy
Wouldn't be a "pvresize /dev/saxy 8G" the correct command to use here?
For future reference who still have interests in this topic, I make a reply to this old question.
The case of the original question is that Volume Group is composed of several Physical Volumes which actually represents physical device (or partition). Therefore you can just shrink volume group by excluding physical volume.
In the case that you want to reduce the size of physical volume itself,
pvresize --setphysicalvolumesize 8G /dev/sdxy
Refer to the man page of pvresize. This command works only when 8G bytes are enough for allocated PE's.
In case that PE allocation is not linear, say, some of PE at the last parts are allocated, pvresize fails. In that case, you have to move PE first, and next pvresize:
pvmove --alloc anywhere -n lvname /dev/sdaxy /dev/sdaxy
Refer to the man page of pvmove. The last two arguments for source and destination should be same in this case, since you are moving PE inside the physical volume.
Next I simply used Gparted, and now it sees free space, and enables modification. I also used system-config-lvm to make the free space inside volume group itself. It's easy and safe to use that tool to reduce the size of logical volume with that tool, but it does not allow to convert the free space to a new partition.