For EC2 instances with two instance volumes, use LVM to combine them into a single /mnt

ec2-lvm.sh

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'
set -x

DEVICE_B="/dev/xvdb"
DEVICE_C="/dev/xvdc"
VG_NAME="ephemeral"
LV_NAME="ephemeral"
DEVICE_LV="/dev/$VG_NAME/$LV_NAME"

install_lvm() {
  apt-get -qq update
  apt-get -qq --yes install lvm2 >/dev/null
}

unmount_mnt() {
  umount /mnt
}

create_logical_volume() {
  pvcreate $DEVICE_B $DEVICE_C
  vgcreate ephemeral $DEVICE_B $DEVICE_C
  lvcreate --extents "100%VG" --name $LV_NAME $VG_NAME
}

format_logical_volume() {
  mke2fs -q -t ext4 -i 8192 "$DEVICE_LV"
}

mount_mnt() {
  sed --in-place=lvmbackup -e "s#${DEVICE_B}#${DEVICE_LV}#" /etc/fstab
  mount /mnt
}

main() {
  install_lvm
  unmount_mnt
  create_logical_volume
  format_logical_volume
  mount_mnt
}

if [ -b "$DEVICE_B" -a -b $DEVICE_C ]; then
  main
else
  echo >&2 "Device $DEVICE_B or $DEVICE_C not present, skipping LVM setup"
fi

Could mke2fs -q -i 8192 /dev/ephemeral/ephemeral be mke2fs -q -i 8192 $DEVICE_LV ?

Fixed, and a few other fixes including specifying ext4 instead of default ext2.