Today I got a new harddisk, so it was finally time for me to have an in depth look at LVM. I use it to combine two harddisks /dev/sdg1 and /dev/sdb1 into one volume group ext_vg which contains one big logical volume /dev/ext_vg/ext. In short, my 250GB and 500GB harddisks are used in one big 750GB ext3 mount point.
I followed the excellent LVM Tutorial, and was surprised how easy everything goes. Here is a short cheat sheet of the important commands (take care to exchange the partition/volume names if you use this):
Installation
I use Ubuntu, so this is a piece of cake:
sudo apt-get install lvm2
There is also a GUI available, but I found the command line tools are very easy to use so there is no real need. I wouldn’t use EVMS as it is not supported in Ubuntu 7.10 and may cause problems
Create a New Filesystem
First I create the physical volume on the partition sdg1, create a new volume group ext_vg that contains this physical volume, and create a new logical volume of size 450GB within the volume group. Finally create the filesystem (disabled reservation space, see Get More Space Out of Your ext3 Partition).
sudo pvcreate /dev/sdg1
sudo vgcreate ext_vg /dev/sdg1
sudo lvcreate -L 450G -n ext ext_vg
sudo mkfs.ext3 -m 0 /dev/ext_vg/ext
Show Status
Each LVM layer has its corresponding command to get information about the metadata:
sudo pvdisplay
sudo vgdisplay
sudo lvdisplay
sudo pvs
sudo vgs
sudo lvs
Mount via fstab
I use the filesystem mainly for data, so allowing just rw is enough (no executables allowed). noatime allows quicker access.
sudo mkdir /media/mega
/dev/ext_vg/ext /media/mega ext3 rw,noatime,user 0 2
mount /media/mega
Resize
It is even possible to do an online resize of the system, wohoo 
This extends the logical volume by 200MB.
sudo lvextend -L +200 /dev/ext_vg/ext
sudo resize2fs -p /dev/ext_vg/ext
You can watch the resize process going on with df -h.
Add Another Partition to the Logical Volume
To add another partition and use up all the available space in the logical volume, first add the physical volume to the volume group, then use pvdisplay to find out the total available number of free PE (add the numbers from the physical volumes), then use lvextend to use up all this available space.
sudo pvcreate /dev/sdb1
sudo vgextend ext_vg /dev/sdb1
sudo pvdisplay
sudo lvextend -l +63602 /dev/ext_vg/ext
sudo resize2fs /dev/ext_vg/ext
That’s it! Any questions? please post!