FFS Backup

For large data sets, home directories, or backups, ZFS has a wide range of supurb features. But if an advanced file system is not available, as FFS on OpenBSD or EXT2/3/4 on Linux can also be used. For backups the goal will be to maintain three copies that guard against hardware failure and human error.

Home directory
Primary backup
Secondary backup

Concat + Copy

Most RAID levels require all storage devices to use the same partition size, but drives of various sizes may be concatenated. Instead of using block-level mirroring (RAID 1), periodic synchronization between the two sets allows for recovery from human error.

  primary: sd1 (1TB) + sd2 (5TB)
secondary: sd3 (1TB) + sd4 (5TB)

OpenBSD RAIDc

In this example disks are concatenated to make to make a larger volume using OpenBSD RAID

fdisk -gy /dev/rsd1c
fdisk -gy /dev/rsd2c

echo 'RAID *' | disklabel -wAT- sd1
echo 'RAID *' | disklabel -wAT- sd2

bioctl -c c -l /dev/sd1a,/dev/sd2a softraid0

fdisk -gy sd4
echo '/ssd *' | disklabel -wAT- sd4
doas newfs /dev/rsd4a

To reconstruct softraid devices, wipe the first megabyte

dd if=/dev/zero of=/dev/sd1c bs=1M count=1
dd if=/dev/zero of=/dev/sd2c bs=1M count=1

Replication

To repliate data from primary to the secondary backup, schedule rsync(1) from /etc/daily.local (run at 01:30 by default)

# abort if file systems are not mounted
df -l /ssd | egrep -q '/ssd$' || exit 1
df -l /hdd | egrep -q '/hdd$' || exit 1

# copy from primary to secondary backup devices
/usr/local/bin/rsync -rlogpt --delete \
  --exclude ".snapshot" \
  /ssd/ /hdd

The delay in synchronizing these two file systems is important, since we do not want mistakes to be replicated immediately.

Inode Snapshots

rsync / openrsync have an important behavior: files are not overwitten in place. Instead temporary files are created and subsequently renamed. This means we can simulate a snapshot by creating hardlinks.

#!/bin/sh -u
# ffs-snapshot.sh

today=$(date +"%Y-%m-%d")

for fs in $(cd /ssd; ls -d *); do
    cd /ssd/$fs
    mkdir -p .snapshot
    # link file, excluding snap cache
    find . -type f ! -path "*/.snapshot/*" | grep -v '/Cache/' | cpio -pmdl /ssd/$fs/.snapshot/$today

    # keep last eight snapshots
    for snap in $(ls -dr /ssd/$fs/.snapshot/* | tail +8); do
        rm -rf /ssd/$fs/.snapshot/$snap
    done
done

cpio(1) often overlooked but it remains standard in all Unix-like distributions, and can combined with other tools in unique ways. Flags used here are

`-p`	Copy files from one location to another
`-m`	Restore modification times
`-d`	Create intermediate directories as needed
`-l`	Links files instead of copying

Because cpio reads a list of files on STDIN we can easily filter out directories that will not be included. ls(1) will display the number of references to each file

$ ls -li /ssd/eradman/{,.snapshot/*}/t14-1260p/.profile
49629792 -rw-r--r--  3 eradman  eradman  488 Nov 22 12:42 /ssd/eradman/.snapshot/2025-12-06/t14-1260p/.profile
49629792 -rw-r--r--  3 eradman  eradman  488 Nov 22 12:42 /ssd/eradman/.snapshot/2025-12-07/t14-1260p/.profile
49629792 -rw-r--r--  3 eradman  eradman  488 Nov 22 12:42 /ssd/eradman//t14-1260p/.profile

Manual Backup

For directories that rarely change a one-time snapshot can allow for recovery after a file has been unlinked

cd ~/Movies
find . -maxdepth 1 -type f | cpio -pmdl $PWD/.snapshot/
sudo chown root:root .snapshot/* .snapshot