In Proxmox 5.x LVM thin is the default local storage for VMs. However I prefer the old method of directory based storage for local storage. You can find this information in the proxmox wiki. These are my terse notes to get the job done.
This should only be done on a new installation of proxmox. In the gui remove the local lvm thin storate.
Type the following to set everything up.
> lvremove pve/data
> lvcreate –name data -l +100%FREE pve
> mkfs.ext4 /dev/pve/data
Add the following to /etc/fstab
/dev/pve/data /var/lib/vz ext4 defaults 0 1
Restart proxmox. Thats it.