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!

	Snapshot Location
Snapshots always reside on the same partition or subvolume that has been “snapshotted”. It is not possible to store snapshots on a different partition or subvolume.

As a result, partitions containing snapshots need to be larger than “normal” partitions. The exact amount strongly depends on the number of snapshots you keep and the amount of data modifications. As a rule of thumb you should consider using twice the size than you normally would.

Freeing space / Disk Usage

In order to free space on a Btrfs partition containing snapshots you need to delete unneeded snapshots rather than files. Older snapshots occupy more space than recent ones. Since the df does not show the correct disk usage on Btrfs file systems, you need to use the command btrfs filesystem df MOUNT_POINT. Displaying the amount of disk space a snapshot allocates is currently not supported by the Btrfs tools. Doing an upgrade from one service pack to another results in snapshots occupying a lot of disk space on the system subvolumes, because a lot of data gets changed (package updates). Manually deleting these snapshots once they are no longer needed is recommended.

Snapper can also be used to create and manage snapshots on thin-provisioned LVM volumes formatted with ext3 or XFS (see Section 4.6, “Using Snapper on Thin-Provisioned LVM Volumes”).

If you set up the root partition with Btrfs during the installation, Snapper—pre-configured for doing rollbacks of YaST or Zypper changes—will automatically be installed. Every time you start a YaST module or a Zypper transaction, two snapshots are created: a “pre-snapshot” capturing the state of the file system before the start of the module and a “post-snapshot” after the module has been finished.

Using the YaST Snapper module or the snapper command line tool, you can undo the changes made by YaST/zypper by restoring files from the “pre-snapshot”. Comparing two snapshots the tools also allow you to see which files have been changed. You can also display the differences between two versions of a file (diff).

Since Linux is a multitasking system, processes other than YaST or Zypper may modify data in the time frame between the pre- and the post-snapshot. If this is the case, completely reverting to the pre-snapshot will also undo these changes by other processes. In most cases this would be unwanted—therefore it is strongly recommended to closely review the changes between two snapshots before starting the rollback. If there are changes from other processes you want to keep, select which files to roll back.

	Limitations
Make sure you know about Snapper's limitations before attempting to use its rollback mechanism. See Section 4.4, “Limitations” for details.

	Storage Time of Snapshots
By default, the last 100 YaST and Zypper snapshots are kept. If this number is exceeded, the oldest snapshot(s) will be deleted.

Apart from the YaST and Zypper snapshots, Snapper creates hourly snapshots of the system partition (/). You can use these backup snapshots to restore files that have accidentally been deleted or modified beyond recovery. By making use of Snapper's diff feature you can also find out which modifications have been made at a certain point of time.

Hourly backup snapshots are of the type Single and are marked with the description timeline. To restore files from these snapshots proceed as described in Procedure 4.1, “Undoing changes using the YaST Snapper module” or Procedure 4.2, “Undoing changes using the snapper command”.

	Storage Time of Snapshots
By default, the first snapshot of the last ten days, months, and years are kept. For details see Example 4.1, “Example time line configuration”.

The way Snapper behaves is defined in a config file that is specific for each partition or Btrfs subvolume. These config files reside under /etc/snapper/configs/. The default config installed with Snapper for the / directory is named root. It creates and manages the YaST and Zypper snapshots as well as the hourly backup snapshot for /.

You may create your own configurations for other partitions formatted with Btrfs or existing subvolumes on a Btrfs partition. In the following example we will set up a Snapper configuration for backing up the Web server data residing on a separate, Btrfs-formatted partition mounted at /srv/www.

You can use either snapper itself or the YaST Snapper module to restore files from these snapshots. In YaST you need to select your Current Configuration, while you need to specify your config for snapper with the global switch -c (e.g. snapper -c myconfig list).

To create a new Snapper configuration, run snapper create-config:

snapper -c www-data create-config
   /srv/www

This command will create a new config file /etc/snapper/config-templates/www-data with reasonable default values (taken from /etc/snapper/config-templates/default).

	Config Defaults
Default values for a new config are taken from /etc/snapper/config-templates/default. To use your own set of defaults, create a copy of this file in the same directory and adjust it to your needs. To use it, specify the -t option with the create-config command: snapper -c www-data create-config -t my_defaults /srv/www

If you have set up the root partition with Btrfs during the installation, Snapper automatically creates hourly snapshots of the system, as well as pre- and post-snapshots for YaST and zypper transactions. Each of these tasks can be disabled as follows:

Each snapshot consists of the snapshot itself and some metadata. When creating a snapshot you also need to specify the metadata. Modifying a snapshot means changing its metadata—you cannot modify its content. The following metadata is available for each snapshot:

Creating a snapshot is done by running snapper create or by clicking Create in the YaST module Snapper. The following examples explain how to create snapshots from the command line. It should be easy to adopt them when using the YaST interface.

	Snapshot Description
You should always specify a meaningful description in order to later be able to identify its purpose. Even more information can be specified via the user data option.

Snapper allows you to modify the description, the cleanup algorithm, and the userdata of a snapshot. All other metadata cannot be changed. The following examples explain how to modify snapshots from the command line. It should be easy to adopt them when using the YaST interface.

To modify a snapshot on the command line, you need to know its number. Use snapper list to display all snapshots and their numbers.

The YaST Snapper module already lists all snapshots. Choose one from the list and click Modify.

To delete a snapshot with the YaST Snapper module, choose a snapshot from the list and click Delete.

To delete a snapshot with the command line tool, you need to know its number. Get it by running snapper list. To delete a snapshot, run snapper delete NUMBER.

	Deleting Snapshot Pairs
When deleting a pre snapshot, you should always delete its corresponding post snapshot (and vice versa).

	Old Snapshots Occupy More Disk Space
If you delete snapshots in order to free space on your hard disk (see Section 4.1.1, “Snapshots and Disk Space” for details), make sure to delete old snapshots first. The older a snapshot is, the more disk space it occupies.

Snapshots are also automatically deleted by a daily cron-job. Refer to Section 4.3.1.2, “Cleanup-algorithms” for details.

There is no mechanism to ensure data consistency when creating snapshot. Whenever a file is written (e.g. a database) at the same time the snapshot is created, it will result in a broken or partly written file. Restoring such a file will cause problems. Therefore it is strongly recommended to always closely review the list of changed files and their diffs. Only restore files that really need to belonging to the action you want to roll back.

Usually /home resides on a separate partition. Such a separate partition is not part of the default configuration for doing YaST rollbacks. Therefore the user's home partition will not be deleted when reverting a user addition using Snapper. It is strongly recommended to use the YaST User and Group Management tool to remove users.

Currently SUSE Linux Enterprise Desktop cannot boot from Btrfs partitions. Therefore a separate partition for /boot is created upon the installation when using Btrfs for the system partition. Since /boot does not support snapshots, the following restrictions apply for YaST/zypper rollbacks: