Full system backup with SquashFS

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 possible.

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

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:

Access Control Lists are not yet supported, and therefore not backed up
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:

squashfs-tools-ng may support SquashFS image files but Windows itself does not support partitionless drives at all.
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.

Wipe out all partitions /dev/sdBN, then the partition table /dev/sdB, with wipefs.
Synchronize all write caches # sync
Inform the OS of partition table changes
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

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

Back up (example script)

-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

$ 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

Retrieve file from backup

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

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