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ZFS Configuration

This text deals with specific ZFS configuration questions for Ansible-NAS. If you are new to ZFS and are looking for the big picture, please read the ZFS overview introduction first.

Just so there is no misunderstanding

Unlike other NAS variants, Ansible-NAS does not install, configure or manage the disks or file systems for you. It doesn't care which file system you use - ZFS, Btrfs, XFS or EXT4, take your pick. Nor does it provides a mechanism for snapshots or disk monitoring. As Tony Stark said to Loki in Avengers: It's all on you.

However, Ansible-NAS has traditionally been used with the powerful ZFS filesystem. Since out of the box support for ZFS on Linux with Ubuntu is comparatively new, this text shows how to set up a simple storage configuration. To paraphrase Nick Fury from Winter Soldier: We do share. We're nice like that.

Using ZFS for Docker containers is currently not covered by this document. See the official Docker ZFS documentation instead.

The obligatory warning

We take no responsibility for any bad thing that might happen if you follow this guide. We strongly suggest you test these procedures in a virtual machine first. Always, always, always backup your data.

The basic setup

For this example, we're assuming two identical spinning rust hard drives for Ansible-NAS storage. These two drives will be mirrored to provide redundancy. The actual Ubuntu system will be on a different drive and is not our concern.

Root on ZFS is possible, but not something that has been tested with Ansible-NAS.

The Ubuntu kernel is already ready for ZFS. We only need the utility package which we install with sudo apt install zfsutils.

Creating a pool

We assume you don't mind totally destroying whatever data might be on your two storage drives, have used a tool such as gparted to remove any existing partitions, and have installed a new GPT partition table on each drive. To create our ZFS pool, we will use a command in this form:

        sudo zpool create -o ashift=<ASHIFT> <NAME> mirror <DRIVE1> <DRIVE2>

The options from simple to complex are:

NAME: ZFS pools traditionally take their names from characters in the The Matrix. The two most common are tank and dozer. Whatever you use, it should be short - think ash, not xenomorph.

DRIVES: The Linux command lsblk will give you a quick overview of the hard drives in the system. However, we don't pass the drive specification in the format /dev/sde because this is not persistent. Instead, always use the output of ls /dev/disk/by-id/ to find the drives' IDs.

ASHIFT: This is required to pass the sector size of the drive to ZFS for optimal performance. You might have to do this by hand because some drives lie: Whereas modern drives have 4k sector sizes (or 8k for many SSDs), they will report 512 bytes because Windows XP can't handle 4k sectors. ZFS tries to catch the liars and use the correct value. However, this sometimes fails, and you have to add it by hand.

The ashift value is a power of two, so we have 9 for 512 bytes, 12 for 4k, and 13 for 8k. You can create a pool without this parameter and then use zdb -C | grep ashift to see what ZFS generated automatically. If it isn't what you think, destroy the pool again and add it manually.

In our pretend case, we use two 3 TB WD Red drives. Listing all drives by ID gives us something like this, but with real serial numbers:


WD Reds have a 4k sector size. The actual command to create the pool would then be:

        sudo zpool create -o ashift=12 tank mirror ata-WDC_WD30EFRX-68EUZN0_WD-WCCFAKESN01 ata-WDC_WD30EFRX-68EUZN0_WD-WCCFAKESN02

Our new pool is named tank and is mirrored. To see information about it, use zpool status tank (no sudo necessary). If you screwed up (usually with ashift), use sudo zpool destroy tank and start over now before it's too late.

Pool and filesystem properties

Pools have properties that apply either to the pool itself or to filesystems created in the pool. You can use the command zpool get all tank to see the pool properties and zfs get all tank to see the filesystem properties. Most default values are perfectly sensible, some you'll want to change. Setting defaults makes life easier when we create our filesystems.

        sudo zpool set autoexpand=on tank
        sudo zfs set atime=off tank
        sudo zfs set compression=lz4 tank

autoexpand=on lets the pool grow when you add larger hard drives. atime=off means that your system won't update a time stamp every time a file is accessed, something which would use a lot of resources. Usually, you don't care. Compression is a no-brainer on modern CPUs and should be on by default (we will discuss exceptions for compressed media files later).

Creating filesystems

To actually store the data, we need filesystems (also known as "datasets"). For our very simple default Ansible-NAS setup, we will create two: One filesystem for movies (movies_root in all.yml) and one for downloads (downloads_root).

Movies (and other large, pre-compressed files)

We first create the basic filesystem:

        sudo zfs create tank/movies

Movie files are usually rather large, already in a compressed format and for security reasons, the files stored there shouldn't be executable. We change the properties of the filesystem accordingly:

        sudo zfs set recordsize=1M tank/movies
        sudo zfs set compression=off tank/movies
        sudo zfs set exec=off tank/movies

The recordsize here is set to the currently largest possible value to increase performance and save storage. Recall that we used ashift during the creation of the pool to match the ZFS block size with the drives' sector size. Records are created out of these blocks. Having larger records reduces the amount of metadata that is required, because various parts of ZFS such as caching and checksums work on this level.

Compression is unnecessary for movie files because they are usually in a compressed format anyway. ZFS is good about recognizing this, and so if you happen to leave compression on as the default for the pool, it won't make much of a difference.

By default, ZFS stores pools directly under the root directory. Also, the filesystems don't have to be listed in /etc/fstab to be mounted. This means that our filesystem will appear as /tank/movies if you don't change anything. We need to change the line in all.yml accordingly:

        movies_root: "/tank/movies"

You can also set a traditional mount point if you wish with the mountpoint property. Setting this to none prevents the file system from being automatically mounted at all.

The filesystems for TV shows, music files and podcasts - all large, pre-compressed files - should probably take the exact same parameters.


For downloads, we can leave most of the default parameters the way they are.

        sudo zfs create tank/downloads
        sudo zfs set exec=off tank/downloads

The recordsize stays the 128 KB default. In all.yml, the new line is

        downloads_root: "/tank/downloads"

Other data

Depending on the use case, you might want to create and tune more filesystems. For example, Bit Torrent, MySQL and Virtual Machines all have known best configurations.

Setting up scrubs

On Ubuntu, scrubs are configured out of the box to run on the second Sunday of every month. See /etc/cron.d/zfsutils-linux to change this.

Email notifications

To have the ZFS demon zed send you emails when there is trouble, you first have to install an email agent such as postfix. In the file /etc/zfs/zed.d/zed.rc, change the three entries:


If zed is not enabled, you might have to run systemctl enable zed. You can test the setup by manually starting a scrub with sudo zpool scrub tank.


Snapshots create a "frozen" version of a filesystem, providing a safe copy of the contents. Correctly configured, they provide good protection against accidental deletion and certain types of attacks such as ransomware. On copy-on-write (COW) filesystems such as ZFS, they are cheap and fast to create. It is very rare that you won't want snapshots.

Snapshots do not replace the need for backups. Nothing replaces the need for backups except more backups.

Managing snapshots by hand

If you have data in a filesystem that never or very rarely changes, it might be easiest to just take a snapshot by hand after every major change. Use the zfs snapshot command with the name of the filesystem combined with an identifier separated by the @ sign. Traditionally, this somehow includes the date of the snapshot, usually in some variant of the ISO 8601 format.

        zfs snapshot tank/[email protected]

To see the list of snapshots in the system, run

        zfs list -t snapshot 

To revert ("roll back") to the previous snapshot, use the zfs rollback command.

        zfs rollback tank/[email protected]

By default, you can only roll back to the most recent snapshot. Anything before then requires trickery outside the scope of this document. Finally, to get rid of a snapshot, use the zfs destroy command.

        zfs destroy tank/[email protected]

Be very careful with destroy. If you leave out the snapshot identifier and only list the filesystem - in our example, tank/movies - the filesystem itself will immediately be destroyed. There will be no confirmation prompt, because ZFS doesn't believe in that sort of thing.

Managing snapshots with Sanoid

Usually, you'll want the process of creating new and deleting old snapshots to be automatic, especially on filesystems that change frequently. One tool for this is sanoid. There are various instructions for setting it up, the following is based on notes from SvennD. For this example, we'll assume we have a single dataset tank/movies that holds, ah, movies.

First, we install sanoid to the /opt directory. This assumes that Perl itself is already installed.

        sudo apt install libconfig-inifiles-perl libcapture-tiny-perl
        cd /opt
        sudo git clone

It is probably easiest to link sanoid to /usr/sbin:

        sudo ln /opt/sanoid/sanoid /usr/sbin/

Then we need to setup the configuration files.

        sudo mkdir /etc/sanoid
        sudo cp /opt/sanoid/sanoid.conf /etc/sanoid/sanoid.conf
        sudo cp /opt/sanoid/sanoid.defaults.conf /etc/sanoid/sanoid.defaults.conf

We don't change the defaults file, but it has to be copied to the folder anyway. Next, we edit the /etc/sanoid/sanoid.conf configuration file in two steps: We design the "templates" and then tell sanoid which filesystems to use it on.

The configuration file included with sanoid contains a "production" template for filesystems that change frequently. For media files, we assume that there is not going to be that much change from day-to-day, and especially there will be very few deletions. We use snapshots because this provides protection against cryptolocker attacks and against accidental deletions.

Again, snapshots, even lots of snapshots, do not replace backups.

For our example, we configure for two hourly snapshots (against "oh crap" deletions), 31 daily, one monthly and one yearly snapshot.

                frequently = 0
                hourly = 2
                daily = 31
                monthly = 1
                yearly = 1
                autosnap = yes
                autoprune = yes

That might seem like a bunch of daily snapshots, but remember, if nothing has changed, a ZFS snapshot is basically free.

Once we have an entry for the template, we assign it to the filesystem.

                use_template = media

Finally, we edit /etc/crontab to run sanoid every five minutes:

        */5 * * * * root /usr/sbin/sanoid --cron

After five minutes, you should see the first snapshots (use zfs list -t snapshot again). The list will look something like this mock example:

NAME                                                USED  AVAIL  REFER  MOUNTPOINT
tank/[email protected]_2019-05-17_13:55:01_yearly      0B      -  1,53G  -
tank/[email protected]_2019-05-17_13:55:01_monthly     0B      -  1,53G  -
tank/[email protected]_2019-05-17_13:55:01_daily       0B      -  1,53G  -

Note that the snapshots use no storage, because we haven't changed anything.

This is a very simple use of sanoid. Other functions include running scripts before and after snapshots, and setups to help with backups. See the included configuration files for examples.