early-boot-aws.service

[Unit]
Description=Create AWS partitions
DefaultDependencies=false
Before=local-fs-pre.target var.mount mnt-data_1.mount -.mount local-fs.target

[Service]
Type=oneshot
ExecStart=/bin/sh /etc/early-boot.sh
RemainAfterExit=yes

[Install]
RequiredBy=local-fs-pre.target

early-boot.sh

#!/bin/sh

if [ ! -b /dev/xvdb1 ]; then
    echo "Creating partition on /dev/xvdb1"
    parted -s -a optimal /dev/xvdb mklabel gpt -- mkpart primary xfs 1 -1
    partprobe /dev/xvdb
    mkfs.xfs /dev/xvdb1
else
    echo "Partition already created on /mnt/data_1"
fi

echo "Mounting partition"
mount /dev/xvdb1 /mnt/data_1

# Create directories on instance store
if [ ! -d /mnt/data_1/var ]; then
    echo "Copying /var"
    mkdir -p -m 777 /mnt/data_1/var
    #cp -a /var/* /mnt/data_1/var/
    rsync -a /var/* /mnt/data_1/var/
    # cd /var
    #find . -depth -print0 | sudo cpio --null --sparse -pvd /mnt/data_1/var
    #mv /var /var.old
    #mkdir -p /var
else
    echo "/var is already copied"
fi

umount /mnt/data_1
exit 0

fstab

none /dev/pts devpts gid=5,mode=620 0 0
none /proc proc defaults 0 0
none /sys sysfs defaults 0 0
/dev/xvdb1 /mnt/data_1 xfs rw,relatime,attr2,inode64,noquota 0 0
/mnt/data_1/var /var none bind,x-systemd.requires=mnt-data_1.mount 0 0

gistfile1.txt

I am trying to create a partition on an unmounted drive and move the /var folder to it. 

The idea I have is: 

1. Create the partition and copy /var data to it  - execute this before any fs is mounted
2. Normal boot continues, picks up the stuff from /etc/fstab and mounts the partitions.