===== Split file system =====
The scenario is: You have one server with LVM and with one large disk, you need split the file system between operating systemvg and appvg, but the problem is that you are using xfs, you can't reduce the file system.

This draft only covers the migration of operating system, you must migrate all app data as you prefer.
==== Pre requisites ====

  * For this task disable SELinux.
  * Two extra disks for systemvg (operating system) and appvg (application files)
  * rsync

==== Procedure ====
Use fdisk to create /boot partition and PV for operating system.
<code>
fdisk /dev/vdb
n
p
1
+1G
n
p
2
enter
w
</code>

Create file system and temporal mount points:
<code>
mkfs.ext4 /dev/vdb1
pvcreate /dev/vdb2
vgcreate rootvg /dev/vdb2
lvcreate -L2G -n swap rootvg
mkswap /dev/mapper/rootvg-swap
lvcreate -l16G -n root rootvg
mkfs.ext4 /dev/mapper/rootvg-root
mkdir /mnt/roottmp
mount /dev/mapper/rootvg-root /mnt/roottmp
mkdir /mnt/roottmp/boot
mount /dev/vdb1 /mnt/roottmp/boot
</code>

Copy all information with rsync, you must change or add app directory in exclude argument:
<code>
rsync -aAXv /boot/* /mnt/roottmp/boot
rsync -aAXv / --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found","/boot/*","/app/*"} /mnt/roottmp
blkid /dev/vdb1
</code>

Update /mnt/roottmp/etc/fstab, you must change /boot UUID, and mount point names and file system type\\

Update /mnt/roottmp/boot/grub2/grub.cfg, same as above.

Mount tmpfs and make a chroot, install grub and update initramfs.
<code>
for i in /dev /proc /sys /run; do mount -B $i /mnt/roottmp$i; done
chroot /mnt/roottmp
grub2-install --recheck /dev/vdb
dracut -f -v
</code>

Reboot!

==== Other media ====
  * https://access.redhat.com/discussions/4217691