The Basic Concepts of Snapper

Snapper has a command-line interface and a YaST interface. Both the command-line interface and the YaST interface can be used interchangeably, allowing you to create, delete and compare snapshots, as well as undo changes made between snapshots.

Using Snapper, you can perform the following tasks:

When a snapshot is created, both the snapshot and the original point to the same blocks in the file system. So, initially a snapshot does not occupy additional disk space. If data in the original file system is modified, changed data blocks are copied while the old data blocks are kept for the snapshot. Therefore, a snapshot occupies the same amount of space as the data modified. So, over time, the amount of space a snapshot allocates constantly grows. As a consequence, deleting files from a Btrfs file system containing snapshots may not free disk space!

Note: Snapshot location

Snapshots always reside on the same partition or subvolume on which the snapshot has been taken. It is not possible to store snapshots on a different partition or subvolume.

As a result, partitions containing snapshots need to be larger than partitions not containing snapshots. The exact amount depends strongly on the number of snapshots you keep and the amount of data modifications. As a rule of thumb, give partitions twice as much space as you normally would. To prevent disks from running out of space, old snapshots are automatically cleaned up.

Snapshots are created for the root partition only, and certain directories are excluded by means of subvolumes. For the list of excluded subvolumes, see Section 3.3, “Subvolumes excluded from snapshots”.

Snapper provides automatic snapshot cleanup algorithms to prevent running out of space on the root partition. These algorithms differentiate between timeline snapshots and numbered snapshots (administration plus installation snapshot pairs). The cleanup behavior can be configured based on the following criteria:

For numbered snapshots, which include administration and installation snapshot pairs, the cleanup is controlled by parameters such as NUMBER_CLEANUP, NUMBER_LIMIT, NUMBER_LIMIT_IMPORTANT, and NUMBER_MIN_AGE. The default values are 2–10 for NUMBER_LIMIT and 4–10 for NUMBER_LIMIT_IMPORTANT, meaning that only the youngest snapshots are kept.

For timeline snapshots, the cleanup is based on the number of snapshots to keep for each type (hourly, daily, weekly, monthly, yearly). For example, the last 24 hourly snapshots, the first daily snapshot from the last seven days, the first snapshot made on the last day of the month for the last twelve months, and so forth. The parameters include TIMELINE_CLEANUP, TIMELINE_MIN_AGE and interval parameters such as TIMELINE_LIMIT_DAILY, TIMELINE_LIMIT_HOURLY and so on.

You can roll back to an existing snapshot at any time by booting from the respective snapshot and making it active afterwards.

Note: Disabling Snapper automatically and manually

If your root partition is smaller than 16 GB, the automatic creation of snapshots as described above is disabled by default. In this case, you can manually create snapshots. Keep an eye on the available disk space.

To disable automatic snapshots even if your root partition is sufficiently sized, disable snapshots manually during the installation in the partition setup step.

When Snapper is configured to operate on root, every Btrfs subvolume is excluded by default.

The default behavior of Snapper is defined in a configuration file that is specific for each partition or Btrfs subvolume. These configuration files reside under /etc/snapper/configs/.

The primary use case for snapshots is to roll back the system to a previous state. Therefore, there are certain subvolumes (directories) for which snapshotting is disabled.

The following list contains directories that are excluded from snapshots. Depending on your product and architecture, not all of them may be available on your system.

Tip

The list of subvolumes is product-specific. To see what subvolumes are created under / and therefore see which directories are excluded from the default snapshots behavior, run:

> sudo btrfs subvolume list /

Note: Why is there a space limit?

When creating a snapshot, no physical data copies are created. A snapshot only consists of pointers to the respective data blocks. As long as the snapshot remains consistent with the current system, it occupies almost no additional disk space (apart from the metadata it contains). However, if a file is modified on the system, the changed data is recorded in the snapshot. Over time, as more changes accumulate and the snapshot diverges from the live system, the snapshot's size increases accordingly.

To prevent disks from running full (and the system becoming inoperable as a result), we recommend having a minimal root file system size. The required size depends on system usage:

• The frequency of snapshot creation

• The duration for which snapshots are retained

• The rate of changes to the system

In general, the more snapshots you have, the longer they are kept, and the more frequently the system changes, the larger the root partition needs to be.