enlarge-proxmox-root-partition.sh

# Copied from http://ttaportal.org/wp-content/uploads/2012/10/7-Reallocation-using-LVM.pdf

##
## Showing the problem: need to reallocate 32GB from /dev/mapper/pve-data to /dev/mapper/pve-root
##

df -h
#    Filesystem Size Used Avail Use% Mounted on
#    /dev/mapper/pve-root 37G 37G 0 100% /
#    tmpfs 2.0G 0 2.0G 0% /lib/init/rw
#    udev 10M 548K 9.5M 6% /dev
#    tmpfs 2.0G 0 2.0G 0 /dev/shm
#    /dev/mapper/pve-data 102G 19G 84G 19% /var/lib/vz
#    /dev/sda1 504M 31M 448M 7% /boot


##
## shrinking /dev/mapper/pve-data
##

# unmount the file system from mount point /var/lib/vz
umount /var/lib/vz

# To check the file system
e2fsck -f /dev/mapper/pve-data 
#    e2fsck 1.41.3 (12-Oct-2008)
#    Pass 1: Checking inodes, blocks, and sizes
#    Pass 2: Checking directory structure
#    Pass 3: Checking directory connectivity
#    Pass 4: Checking reference counts
#    Pass 5: Checking group summary information
#    /dev/mapper/pve-data: 20/4587520 files (0.0% non-contiguous), 333981/18350080 blocks

# shrink the file system from 102G to 70G
resize2fs /dev/mapper/pve-data 70G
#   Resizing the filesystem on /dev/mapper/pve-data to 18350080 (4k) blocks
#   The filesystem on /dev/mapper/pve-data is now 18350080 blocks long

# reduce the logical volume /dev/mapper/pve-data with 32GB (102 – 32 = 70 GB)
lvreduce -L-32G /dev/mapper/pve-data 

# remount the file system mounted on /var/lib/vz
mount /var/lib/vz



## 
## extend /dev/mapper/pve-root
##

# extend the logical volume to fill 100% of the free space
lvextend -l +100% FREE /dev/mapper/pve-root

# expand the filesystem (by default performs on-line resizing!!!)
resize2fs /dev/mapper/pve-root


##
## checking the output
##

df -h
#    Filesystem Size Used Avail Use% Mounted on
#    /dev/mapper/pve-root 73G 31G 39G 45% /
#    tmpfs 2.0G 0 2.0G 0% /lib/init/rw
#    udev 10M 548K 9.5M 6% /dev
#    tmpfs 2.0G 0 2.0G 0% /dev/shm
#    /dev/sda1 504M 31M 448M 7% /boot
#    /dev/mapper/pve-data 69G 404M 69G 1% /var/lib/vz

@brandinarsenault, it turns out that it was actually an issue with how I setup my containers and VMs. I used Proxmox's storage defaults when I first set everything up, and apparently that's what ended up giving me the above issue. This thread should give you everything you need, provided it's actually the same issue I was having.