Full system backup with SquashFS

Related articles

• Full system backup with tar

Compressed backup archives of whole filesystems can be made in the SquashFS format. Backup and retrieval takes relatively less time with its random access capability, but any kind of modification other than appending is not yet impossible.

Device File	Description
/dev/sdB	Backup Drive
/dev/sdL	Live Medium
/dev/sdSRC	Drive to Be Backed Up

Warning: SquashFS does not support Access Control Lists.[1]

Pros and cons

Pros:

• No partitioning, no guessing how much size is required
• SquashFS should be supported by most rescue disks
• Random access^{[citation needed]}, with a plain mount
• Duplicate files are removed by default (unless -no-duplicates flag is on)

Cons:

• All ACLs are lost
• Not accessible from Windows¹
• Appendable but impossible to remove things from it
• To use the disk for some other purposes, you have to destroy the backup²

Note:

1. squashfs-tools-ng may support SquashFS image files but Windows itself does not support partitionless drives at all.
2. Do not grab the lifebuoy when you need a swim ring. File sharing and backup should be in separate disks. Please buy adequate backup hardware.

Prepare the backup drive

Warning:

• All data on the backup drive will be lost.
• All data on machine may be lost if an incorrect device file is specified.

1. Wipe out all partitions /dev/sdBN, then the partition table /dev/sdB, with wipefs.
2. Synchronize all write caches # sync
3. Inform the OS of partition table changes # partprobe
4. Check for bad blocks:
   • If the backup drive is an SSD, use S.M.A.R.T..
   • If the backup drive is an HDD, use badblocks.

Prepare a live medium

Installation guide#Prepare an installation medium or archiso to /dev/sdL

Note: Creating SquashFS filesystem requires squashfs-tools, which is included in the official Arch Linux installation image.

Back up

Installation guide#Boot the live environment

Tip: If you do not have enough USB ports for both the backup drive and live medium:

1. Boot the live medium
2. When the boot loader menu appears, press 'e'
3. Append copytoram to kernel parameters
4. # udisksctl power-off -b /dev/sdL (requires archiso releng with udisks2 in packages.x86_64)
5. Make sure the live medium stops spinning and disconnect it

Mount filesystems you would like to backup. (e.g. /dev/sdSRC to /mnt)

# fsck /dev/sdSRC
# mount -o ro /dev/sdSRC /mnt

Back up (example script)

Warning:

• All data on the backup drive will be lost.
• All data on machine may be lost if an incorrect device file is specified.

# mksquashfs /mnt /dev/sdB \
  -not-reproducible \
  -xattrs \
  -wildcards \
  -noappend \
  -progress \
  -mem 5G \
  -e \
    var/cache/pacman/pkg \
    var/lib/pacman/sync \
    var/log/journal \
    boot/efi \
    boot/grub \
    boot/initramfs-linux"*".img \
    boot/vmlinuz-linux

-not-reproducible

Slightly increase backup speed

-noappend

Overwrite

-mem

RAM granted to mksquashfs

-e

List of exclude dirs/files

(Optional) Test backup in QEMU

Grant permission

$ sudo chown $USER:$USER /dev/sdB

Launch QEMU

Note: Kernel and initramfs from the host machine are used.

$ qemu-system-x86_64 \
  -kernel /boot/vmlinuz-linux \
  -append "root=/dev/sdB ro loglevel=3 init=/usr/lib/systemd/systemd" \
  -initrd /boot/initramfs-linux-fallback.img \
  -drive file=/dev/sdB,index=0,media=disk,format=raw \
  -m virtual_machine_ram_in_MiB \
  --enable-kvm \
  -cpu host

(Optional) Inspect backup

Get backup date

# date --date=@"$(unsquashfs -mkfs-time /dev/sdB)"

More superblock information

# unsquashfs -stat /dev/sdB

Checksum

# COUNT="$(unsquashfs -stat /dev/sdB | grep -F 'Filesystem size' | cut -d' ' -f3)"
# echo "$COUNT Byte"
# numfmt --to=iec-i "$COUNT"
# head -c "$COUNT" /dev/sdB | sha1sum | tee -a sha1sum.txt

Retrieve file from backup

# mount /dev/sdB /mnt
# cp /mnt/path/to/file /path/to/destination

Restore backup

Note:

To make system bootable after restore:

• fix fstab
• arch-chroot
  • regenerate the initramfs
  • install and configure boot loader