A snapshot is like copying a the files in the snapshot to another location, while the original files remain as is and can be written to or read from.

As an example, if you have a folder, let’s say /important/secret/stuff, and you want to take a backup of it. You have to make sure that nothing is writing to those files while the backup is running, otherwise the backup risk being corrupted. (This is because if the backup starts to read the file, and halfway through the first part of the file changes, you now have a backuped file that half the old and half the new file).

With a snapshop, you can “copy” /important/secret/stuff to some other location, run a backup reading from this other location and then remove it. Any changes to the original files will not affect the backup since the backup is reading from the “copied” files (snapshot).

So what you do is take a snapshot before running the backup, have the backup read from the snapshot and when it is complete, remove the snapshot.

Well there’s software that can make it easier. Just an option. For ZFS specifically there’s zfs_autobackup which I’ve got experience with and I haven’t tried this but just found out about a Web UI that works with it: https://github.com/natankeddem/bale

Out of curiosity, how are you doing the drive imaging?

I wrote a script that is fired by a cron job once a month:

sudo nano /usr/local/bin/backup_drive.sh

# Directory to store backups

BACKUP_DIR="/mnt/myhdd"

# Drive to backup

DRIVE="/dev/sdX"

# Create a timestamp

TIMESTAMP=$(date +"%Y%m%d")

# Filename for the new image

NEW_IMAGE="$BACKUP_DIR/my_drive_image_$TIMESTAMP.img"

# Create the image

sudo dd if=$DRIVE of=$NEW_IMAGE bs=4M status=progress

# Keep the last 3 images

ls -tp $BACKUP_DIR/my_drive_image_*.img | grep -v '/$' | tail -n +4 | xargs -I {} rm -- {}

Make the script executable:

sudo chmod +x /usr/local/bin/backup_drive.sh

Create Cron:

sudo crontab -e

0 3 1 * * /usr/local/bin/backup_drive.sh

Verify cron:

sudo crontab -l

I’m sure it could be written differently or that there is a better way to do the imaging, but that is the extent of my notes, and it works for me. As with any code you find on the internet, it is wise to evaluate before dropping on a production server.