ZFS datasets
ZFS datasets are a powerful and flexible organizational tool that let you easily and quickly structure your data, monitor size over time, and take backups. Learn more about how to create and manage ZFS datasets in this article.

We'll use Ubuntu as the operating system and sanoid and syncoid for easily sending and receiving ZFS snapshots. However, any flavor of Linux or FreeBSD should work too, though the installation steps will differ. To install ZFS on Ubuntu, simply run sudo apt-get install zfsutils-linux.

You can give ZFS a try without formatting a real disk; simply create an image file and format it as ZFS:

1# create a 1000MB image file
2dd if=/dev/zero of=/tmp/zfs.img bs=1M count=1000
4# create a zpool called "food" on this image file
5zpool create food /tmp/zfs.img

What is a dataset?

A traditional filesystem like ext4 runs inside one disk partition. This is a simple structure, but inflexible if the growth of your data changes over time and you later want to rearrange your data. For example, imagine you created 2 partitions, a 10GB one for vegetables and a 5GB one for fruit. Perhaps you initially expect your vegetable data to grow much more quickly, but what if the reverse happens? You may end up with 5GB of fruit data, and now the partition has to be resized in order to make room. While this operation is possible, it will probably involve moving other data on the disk somewhere else and require the fruit partition to be unmounted temporarily. Using ZFS datasets, you can dynamically move the space where it is needed, no resizing or shuffling of data required.

Think of a zpool as a collection of dynamic filesystems (aka datasets) that can be nested within each other, resized, snapshotted, etc. For example, a zpool of food could look like this:

 1$ zfs list -r food
 2NAME                       USED  AVAIL     REFER  MOUNTPOINT
 3food                       388K   832M       24K  /food
 4food/fruit                  96K   832M       24K  /food/fruit
 5food/fruit/apples           24K   832M       24K  /food/fruit/apples
 6food/fruit/bananas          24K   832M       24K  /food/fruit/bananas
 7food/fruit/oranges          24K   832M       24K  /food/fruit/oranges
 8food/vegetables            120K   832M       24K  /food/vegetables
 9food/vegetables/broccoli    24K   832M       24K  /food/vegetables/broccoli
10food/vegetables/carrots     24K   832M       24K  /food/vegetables/carrots
11food/vegetables/celery      24K   832M       24K  /food/vegetables/celery
12food/vegetables/tomatoes    24K   832M       24K  /food/vegetables/tomatoes

If you wanted to manage your apples, you can cd /food/fruit/apples and similar for each of the other foods. If you later decide that tomatoes are really a fruit, you can move them simply with zfs rename food/vegetables/tomatoes food/fruit/tomatoes.

How do you configure a dataset?

Each dataset has a number of properties that you can get or set on it using zfs get and zfs set respectively. Some examples of what you can do with properties:

  • Compression: Save space on the data stored in your dataset by enabling compression; there are a number of algorithms to choose from, and you can recursively enable compression on child datasets by simply setting it on the parent dataset. Note that changing compression settings doesn't affect existing data in the dataset, only new data written from then on. For an example, let's enable compression for all vegetable data:
 1$ zfs set compression=lz4 food/vegetables
 2$ zfs get compression -r food
 3NAME                      PROPERTY     VALUE           SOURCE
 4food                      compression  off             default
 5food/fruit                compression  off             default
 6food/fruit/apples         compression  off             default
 7food/fruit/bananas        compression  off             default
 8food/fruit/oranges        compression  off             default
 9food/vegetables           compression  lz4             local
10food/vegetables/broccoli  compression  lz4             inherited from food/vegetables
11food/vegetables/carrots   compression  lz4             inherited from food/vegetables
12food/vegetables/celery    compression  lz4             inherited from food/vegetables
13food/vegetables/tomatoes  compression  lz4             inherited from food/vegetables
  • Encryption: You can enable ZFS encryption on a dataset easily using a password
  • Quotas: If you want to prevent a particular dataset from growing larger than a certain size, you can use a quota.
  • Mountpoint: You can configure where a dataset is mounted, even in a directory completely unrelated to other datasets in the pool, using zfs set mountpoint=/path/to/the/new/mountpoint the/dataset/name.
  • Custom Information: You can even store custom information about a dataset in User Properties. For example, to store the color of the apples:
1$ zfs set custom:color=red food/fruit/apples
2$ zfs get custom:color food/fruit/apples
3NAME               PROPERTY      VALUE         SOURCE
4food/fruit/apples  custom:color  red           local

How do you backup datasets?

Even more powerful than any of the above features is ZFS's ability to take, send, and delete snapshots. While you can do this directly with ZFS commands, I highly recommend the combination of sanoid and syncoid to fully automate this process.


Sanoid lets you define a schedule on which to take the snapshots and how many of them to keep around (e.g. 12 hourly, 7 daily, 4 weekly, 6 monthly, and 1 yearly):

2    recursive = yes
3    hourly = 12
4    daily = 7
5    weekly = 4
6    monthly = 6
7    yearly = 1

It will automatically clean up extraneous old snapshots as they accumulate, and take new ones on the prescribed schedule; simply configure a cron job to run sanoid frequently:

10 * * * * /usr/local/bin/sanoid --cron


Syncoid lets you easily copy these snapshots to another server, for example to make backups. The beauty of syncoid is the syntax is as easy to use as cp or rsync, making it very simple even if you're recursively syncing a whole zpool of datasets and all their snapshots. For example, to sync all of this food data to a backup server called mybackups:

1syncoid -r food user@mybackups:food

Again, you should configure a cron job to run it regularly.


ZFS is a powerful filesystem with a long history and proven track record. Its combination of features and simplicity makes organizing and backing up your data easy and gives you flexibility to adapt as your needs change over time.

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