How to change default settings and make software raid (mirror and stripe) with GPT.

While booting installation:

linux ext4 hdsize=10 maxroot=5 swapsize=1 maxvz=1

http://www.jamescoyle.net/how-to/261-proxmox-advanced-install-settings

In case your partitions were used with raid you should wipe them (dry :]):

wipefs -a /dev/sdb1
# or mdadm --zero-superblock /dev/sdb[1-4]

apt-get -y update
apt-get -y dist-upgrade
START_AT=`parted /dev/sda p | tail -n 2 | head -n 1 | awk -F " " '{print $3}'`
parted /dev/sda mkpart primary $START_AT 100% 
apt-get install xfsprogs -y
apt-get -y install mdadm

sgdisk -b=/tmp/partitions /dev/sda
sgdisk -l=/tmp/partitions /dev/sdb

dd if=/dev/sda1 of=/dev/sdb1
sgdisk -t 2:fd00 /dev/sdb
sgdisk -t 3:fd00 /dev/sdb
sgdisk -t 4:fd00 /dev/sdb
sgdisk -t 4:fd00 /dev/sda

# To create a "degraded" array in which some devices are missing, simply give the word "missing" in 
# place of a device name.  This will cause mdadm to leave the corresponding slot in the array empty.
# For a RAID4 or RAID5 array at most one slot can be "missing"; for a RAID6 array at most two slots.  # For a RAID1 array, only one real device needs to be given.  All of the others can be "missing".
yes | mdadm --create /dev/md0 --level=1 --raid-disks=2 missing /dev/sdb2
yes | mdadm --create /dev/md1 --level=1 --raid-disks=2 missing /dev/sdb3
yes | mdadm --create /dev/md2 --level=0 --raid-disks=2 --chunk=256 /dev/sda4 /dev/sdb4
/usr/share/mdadm/mkconf > /etc/mdadm/mdadm.conf
mkfs.ext4 /dev/md0
mkfs.ext4 /dev/md1
# xfsprogs and the mkfs.xfs utility automatically select the best stripe size and stripe width for underlying devices that support it, such as Linux software RAID devices.
mkfs.xfs -f /dev/md2
mkdir /mnt/md0
mount /dev/md0 /mnt/md0
cp -ax /boot/* /mnt/md0
sed -i  's/UUID=.*\s\/boot/\/dev\/md0 \/boot/' /etc/fstab
reboot

After reboot

echo 'GRUB_DISABLE_LINUX_UUID=true' >> /etc/default/grub
echo 'GRUB_PRELOAD_MODULES="raid dmraid"' >> /etc/default/grub
echo raid1 >> /etc/modules
echo raid1 >> /etc/initramfs-tools/modules
grub-install /dev/sda --recheck
grub-install /dev/sdb --recheck
update-grub
update-initramfs -u

sgdisk -t 2:fd00 /dev/sda
mdadm --add /dev/md0 /dev/sda2

pvcreate /dev/md1
vgextend pve /dev/md1
pvmove /dev/sda3 /dev/md1
vgreduce pve /dev/sda3
pvremove /dev/sda3
sgdisk -t 3:fd00 /dev/sda
mdadm --add /dev/md1 /dev/sda3

# voila...
lsblk -o NAME,MAJ:MIN,RM,SIZE,TYPE,FSTYPE,MOUNTPOINT,UUID

Sources:
http://www.anchor.com.au/blog/2012/10/the-difference-between-booting-mbr-and-gpt-with-grub/
http://alexunil.net/2013/09/12/proxmox-3-1-auf-softraid-mit-gpt/
http://boffblog.wordpress.com/2013/08/22/how-to-install-proxmox-ve-3-0-on-software-raid/