A quick update from my side. Context: today I got online a dedicated server installed with physical partition scheme instead of LVM. There were 3 partitions:
/boot (ext4) - 512M
/ (ext4) - 730G
swap - 8G
Due to the nature of not having console access the final goal was the convert the existing root partition to LVM.
Considering the ext4 is not shrinkable the only way was to reuse the swap partition as temporary root. I also decided to set up the temporary root with LVM to be sure the process can work in the right way.
First turned out the swap:
Then resized the partition via parted (originally it was started from 742 to 750):
resize 3 742 744
and created a partition for the LVM:
mkpart primary ext2 744 750
set 4 lvm on
PV/VG/LV/filesystem creation for the temp root:
vgcreate VolGroup00 /dev/sda4
lvcreate -L 5.73G -n tmproot VolGroup00
Next step was to copy the root to the temporary place:
mount /dev/VolGroup00/tmproot /media
rsync -ravzxq / /media/
Once everything was there then the entry for the root filesystem in /media/etc/fstab had to be changed as well:
/dev/VolGroup00/tmproot / ext4 defaults 1 1
Almost there, the last and let say the most unwanted part without console access was to modify the /boot/grub/grub.conf:
title CentOS (2.6.32-279.22.1.el6.x86_64)
kernel /vmlinuz-2.6.32-279.22.1.el6.x86_64 ro root=UUID=e769af21-d9e1-455f-a6a7-7a9c84d8cbea rd_NO_LUKS LANG=en_US.UTF-8 KEYBOARDTYPE=pc KEYTABLE=hu rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_NO_LVM rd_NO_DM rhgb quiet
title CentOS (2.6.32-279.22.1.el6.x86_64) LVM
kernel /vmlinuz-2.6.32-279.22.1.el6.x86_64 ro LANG=en_US.UTF-8 KEYBOARDTYPE=pc KEYTABLE=hu SYSFONT=latarcyrheb-sun16 crashkernel=auto dolvm root=/dev/mapper/VolGroup00-tmproot panic=10
#rd_NO_DM rd_NO_MD rd_NO_LUKS
Just to be on the safe side: insmod lvm was added along with the dolvm parameter for the kernel and the root path was also changed to root=/dev/mapper/VolGroup00-tmproot. Important to use the /dev/mapper/ path at this time. As a safty deposit I also added the panic=10 parameter and didn't change the default boot entry in the header. Instead of that I went for a try with telling the grub to boot with the new setting only one time and in case of failure the original entry could work:
savedefault --default=1 --once
It was OK for me at the first time so I repeated the whole procedure with creating a new volume group on top of the original root partition and finally I got the root at the right place using LVM.
Hope this helps.