The recent 1.0.1-beta versions are now based on unionfs. The root filesystem of the livecd is the union of two filesystems: first it shows the contents of the read-only squash filesystem stored in sysrcd.dat that contains all the programs. The second filesystem is a writeable tmpfs filesystem that allows to you to store changes. This switch to unionfs gives the users the ability to edit all the files of the root filesystem of the livecd at runtime. It also saves some memory.

To know why it’s an improvement it’s necessary to understand how it worked before. Until SystemRescueCd-1.0.0, the root filesystems was made of two kinds of directories. Most the directories that store programs (/bin, /lib, /usr, /sbin, /opt, …). These read-only directories were just links to subdirectories of /mnt/livecd that is where the squash filesystem is mounted. Thus all these directories were read-only, and it was not possible to add a file or to change anything. The other directories had to be read-write: /root, /etc, /var. Since it was necessary to let the user write in these directories, they were copied into ram at boot time. For instance the /var/lib/clamav directory stores the clamav antivirus definition files. Since these files may be updated they had to be stored in memory and about 10MB were wasted when the users don’t need it.

Now all the files of the livecd can be edited, removed, changed at runtime. This means it’s easier to make simple changes on the livecd at runtime. For instance it lets you edit a script in the system for debugging purposes. You can also install your own scripts or programs at runtime on the livecd. When a file is not changed by the user, it does not use any memory. Thus the clamav virus definition files will not use memory until the user runs the freshclam program that downloads a newer version from the official website. So the unionfs saves memory.