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Administration Guide / Common tasks / Read-only file system
Applies to SUSE Linux Enterprise Micro 5.3

1 Read-only file system

  

This chapter focuses on the characteristics of the read-only file system that is used by SLE Micro.

SLE Micro was designed to use a read-only root file system. This means that after the deployment is complete, you are not able to perform direct modifications to the root file system, e.g. by using zypper. Instead, SUSE Linux Enterprise Micro introduces the concept of transactional updates which enables you to modify your system and keep it up to date.

The key features of transactional updates are the following:

  • They are atomic - the update is applied only if it completes successfully.

  • Changes are applied in a separate snapshot and so do not influence the running system.

  • Changes can easily be rolled back.

Each time you call the transactional-update command to change your system—either to install a package, perform an update or apply a patch—the following actions take place:

Procedure 1.1: Modifying the root file system
  

  1. A new read-write snapshot is created from your current root file system, or from a snapshot that you specified.

  2. All changes are applied (updates, patches or package installation).

  3. The snapshot is switched back to read-only mode.

  4. If the changes were applied successfully, the new root file system snapshot is set as default.

  5. After rebooting, the system boots into the new snapshot.

    Note
    Note

    Bear in mind that without rebooting your system, the changes will not be applied.

Warning
Warning

In case you do not reboot your machine before performing further changes, the transactional-update command will create a new snapshot from the current root file system. This means that you will end up with several parallel snapshots, each including that particular change but not changes from the other invocations of the command. After reboot, the most recently created snapshot will be used as your new root file system, and it will not include changes done in the previous snapshots.

1.1 /etc on a read-only file system

  

Even though /etc is part of the read-only file system, using an OverlayFS layer on this directory enables you to write to this directory. All modifications that you performed on the content of /etc are written to the /var/lib/overlay/SNAPSHOT_NUMBER/etc. Each snapshot has one associated OverlayFS directory.

Whenever a new snapshot is created (for example, as a result of a system update), the content of /etc is synchronized and used as a base in the new snapshot. In the OverlayFS terminology, the current snapshot's /etc is mounted as lowerdir. The new snapshot's /etc is mounted as upperdir. If there were no changes in the upperdir /etc, any changes performed to the lowerdir are visible to the upperdir. Therefore, the new snapshot also contains the changes from the current snapshot's /etc.

Important
Important: Concurrent modification of lowerdir and upperdir

If /etc in both snapshots is modified, only the changes in the new snapshot (upperdir) persist. Changes made to the current snapshot (lowerdir) are not synchronized to the new snapshot. Therefore, we do not recommend changing /etc after a new snapshot has been created and the system has not been rebooted. However, you can still find the changes in the /var/lib/overlay/ directory for the snapshot in which the changes were performed.

Note
Note: Using the --continue option of the transactional-update command

If you use the --continue option of the transactional-update command when performing changes to the file system, all /etc directory layers created by each separate run of transactional-update, except for the one in the newest snapshot, are synchronized to the lowerdir (the lowerdir can have several mount points).