Creating a backing store


Overview

SystemRescue is based on a read-only compressed file system. All the files of the system are stored in a large squashfs file system image file. As a consequence, changes you make on the system are not saved, and they are lost when you reboot (except what you do on the other file systems that you may have mounted).

The root file system in SystemRescue an overlayfs since version 6.0. Hence all changes made on system files are allowed and stored in memory. It allows you to change a system file, for instance you can replace a program with your own version, or you can make configuration changes in applications such as Firefox.

Users who want to keep their changes in the system files can create a custom SystemRescue media. It is very convenient when you want to add new programs to the system, but it is not comfortable if you often have to change files in the system. You do not want to make a new customized version everyday.

That is why SystemRescue provides the backing-store feature. A backing-store is a loopback file system stored on an USB-stick or on an hard drive, which contains all the files of the system that have been changed. The modifications are saved to the backing-store every time you edit a file, when you create a new directory, or when the system writes or deletes a file for any other reason. As a consequence, it allows you to keep your configuration changes: you can add your bookmarks and extensions to Firefox, and they will still be there when you reboot SystemRescue, as long as the same backing-store is loaded.

Backing stores are not compatible between SystemRescueCd-5.x and SystemRescue since version 6.0 as they use different union file systems (aufs vs overlayfs).

Using a backing store with SystemRescue-6.x and newer

Since version 6.0 SystemRescue uses overlayfs instead of aufs. You have to use the cow_xxxx boot parameters in order to persist changes.

You need to have a Linux file system where SystemRescue is allowed to store its changes. It can be any Linux file system such as ext4, xfs or btrfs. You can either create a dedicated file system to store these changes or you can reuse an existing file system as changes will be isolated in a directory. You need provide SystemRescue an option on the boot command line (or in the boot loader configuration file) so it can identify the file system where to store these changes. The recommended method is to specify the file system label but this is not the only way.

You then need to boot SystemRescue with option cow_label=xxxx where xxxx is the label of a Linux file system where you want changes to be stored. For example you use cow_label=boot if your /boot file system is labelled boot. This is similar to cow_device=/dev/disk/by-label/boot.

Changes will be stored in a directory prefixed with persistent. You can override this default using the cow_directory=xxxx option.

You can read archiso README.bootparams if you want more details about the boot options supported in SystemRescue.