shrink_to_export.sh

#!/bin/sh

VM_SYS_CAPACITY=$(df -h / | tail -n 1 | awk '{print $2}')

echo "[Hit CTRL+C right NOW! if you previously did NOT read that WARNING]"
sleep 2
echo
echo
echo
echo
echo "
This script is a helper when shrinking or recovering unused free guest VM disk space.

Note: Hypervisors like Proxmox allows to discard unused free space, QNAP VM clone functions work very similar and in fact
      is equal to: 'qemu-img convert -p -O qcow2 -o compat=1.1,lazy_refcounts ...'

Requirements:
1) Script will perform quick disk defragmentaion to move data from end parts of the disk volume.
   Recommended: Before doing this operation its recommended to manually stop as much services as possible.

once defragmentation is done....
2) free space on the guest will be filled with 0 bytes:
   That means:
   - On the host you have at least: ${VM_SYS_CAPACITY} free space on the datastore - OTHERWISE HIT CTRL+C
   - Critical services like MySQL/MariaDB has to be stopped for safety reasons
     (to prevent crash & corrupt DB disk image will run out of free space)

---- [Hit CTRL+C to ABORT]: 10 seconds left ...."

sleep 10

stop_services(){
    echo Stop critical services....
    service monitorix stop 2>/dev/null 1>/dev/null
    service grafana-server stop 2>/dev/null 1>/dev/null
    service memcached stop 2>/dev/null 1>/dev/null
    service redis-server stop 2>/dev/null 1>/dev/null
    systemctl stop kibana 2>/dev/null 1>/dev/null
    systemctl stop elasticsearch 2>/dev/null 1>/dev/null
    service mysql stop 2>/dev/null 1>/dev/null
    service apache2 stop 2>/dev/null 1>/dev/null
}

start_services(){
    echo Start services once again....
    service mysql start 2>/dev/null 1>/dev/null
    systemctl start elasticsearch 2>/dev/null 1>/dev/null
    systemctl start kibana 2>/dev/null 1>/dev/null
    service apache2 start 2>/dev/null 1>/dev/null
    service monitorix start 2>/dev/null 1>/dev/null
    service grafana-server start 2>/dev/null 1>/dev/null
    service memcached start 2>/dev/null 1>/dev/null
    service redis-server start 2>/dev/null 1>/dev/null
}

drop_cache(){
    echo Drop and flush cache...

    sync
    # drop cache
    sysctl -w vm.drop_caches=3

    # drop memory cache
    echo 3 > /proc/sys/vm/drop_caches
    echo 1 > /proc/sys/vm/compact_memory
    # sync once again
    sync
    # make sure to sync once again
    sync
    # and again for last time
    sync
}

stop_services

drop_cache

# install dependencies
apt update 2>/dev/null 1>/dev/null
apt install fstrim e4defrag 2>/dev/null 1>/dev/null

# do defrag
echo Defragmenting data:
e4defrag /

drop_cache

# fill up hdd with zeros
dd if=/dev/zero of=/zero bs=64k
drop_cache
# cleanup
rm -f /zero

drop_cache

echo Lets trim all devices
fstrim -a -v 2>/dev/null 1>/dev/null


echo Lets trim all devices last time...
fstrim -a -v 2>/dev/null 1>/dev/null

echo Done
echo Please shutdown VM and clone to new VM.

echo [Hit CTRL+C to abort shut-down procedure]
sleep 10
shutdown -P 0