Dennis Lee

Snapshots

The root directory of a ZFS filesystem contains an invisible folder .zfs/. This differs from a regular “hidden” folder in that it doesn’t show up with ls -A and other such operations.¹

Inside .zfs/snapshot/, every available snapshot is represented in a directory. A snapshot is just a complete representation of the entire file system as it existed at a particular point in time. So you can recover an earlier version of something just by pulling it from one of these snapshots.

I won’t go into the details here, but snapshots are very computationally cheap, unlike if you tried to recreate something like this with cp(1) and rsync(1) or whatever. So it’s advised to create many of them and create them often to maximize their usefulness.

Debian has a couple scripts in its official repositories that completely automate periodically creating new snapshots and removing older ones: zfs-auto-snapshot(8) in contrib and zfsnap in main. Personally, I use the former.

The zfs-auto-snapshot(8) package installs its default cron jobs automatically, so it’s not necessary to configure anything. It does frequent snapshots every 15 minutes (keeping the 4 most recent ones) and also hourly, daily, weekly, and monthly ones. It seems like zfsnap does not come with cron jobs.

Degraded arrays

In a mirrored layout, ZFS will carry on if a disk fails. Here’s what I did in my testing of this:

cd data
echo 'This is a file.' > file.txt
cat file.txt

This is a file.

shutdown -h now
# Unplug one of the hard drives
# Power on
cd data
cat file.txt

This is a file.

echo 'This is a change I made.' >> file.txt
cat file.txt

This is a file.
This is a change I made.

shutdown -h now
# Reattach the unplugged hard drive
# Power on
cd data
cat file.txt

This is a file.
This is a change I made.