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Revision as of 10:00, 12 July 2011
Move all to /usr
Summary
Provide a way of mounting /usr read-only and share it between multiple hosts to save maintenance and space.
/usr on its own filesystem is useful in custom setups. But instead of the Unix way to (almost randomly) split-off tools from /usr and put them in /, and require more and more tools to move to /, we today just expect /usr to be mounted from inside the initramfs, to be available before 'init' starts. What /bin and /sbin was for Unix is the initramfs for Linux. An initramfs that supports to mount /usr on top of /, before it starts 'init', makes all current setups work properly.
Owner
- Name: Harald Hoyer
- Email: harald@redhat.com
Current status
- Targeted release: Fedora 17
- Last updated: (DATE)
- Percentage of completion: 0%
Detailed Description
There is no way to reliably bring up a modern system with an empty /usr, there are two alternatives to fix it: copy /usr back to the rootfs or use an initramfs which can hide the split-off from the system.
Historically /bin, /sbin, /lib had the purpose to contain the utilities to mount /usr. This role can now be taken by the initramfs. Because the initramfs knows, where to find the root partition (which includes /etc), it can parse /etc/fstab and other configuration files and mount /usr before it finally switches the root partition and executes /usr/bin/init. From this point on init mounts the remaining partitions in /etc/fstab and the system starts as usual.
The long-term plan is to clean up the mess and confusion the current split of / vs. /usr has created. All tools will move back to /usr where they belong, and the rootfs will only contain compat-symlinks into /usr. Almost the entire system installed by packages will reside in /usr. This will split all non-host specific data to /usr. /usr can then be seen as the Unix System Resources partition (/System), which defines the base operating system (e.g. F18 or RHEL-7).
This new /usr could be mounted read-only by default, while the rootfs is read-write and contains only empty mount points, compat-symlinks to /usr and the host-specific data like /etc, /root, /srv. Compared to today's setups, the rootfs will be very small. The new /usr could also easily be shared read-only across several systems, and it would contain almost the entire system. Such setups are more efficient, can optionally provide a lot more security, are more flexible, provide more sane options for custom setups, and are much simpler to setup and maintain.
The remaining non-volatile top level directories are host specific:
- /boot - data to boot the machine (bootloader, kernel and initramfs image)
- /var - host specific variable data
- /home - user data
- /root - user data
- /etc - host specific configuration data
- /opt - host specific non-base OS apps
- /srv - host specific contents to be served
Benefit to Fedora
Clear separation of operating system and host specific resources. /usr can be read-only and shareable.
Scope
The ability to share /usr is especially useful for clusters and virtual machines. The ability to mount /usr read-only (e.g. on read-only media) adds to the security of the machine.
How To Test
User Experience
- less toplevel directories
Dependencies
- initramfs (dracut)
- selinux
- repackaging of packages with content in /bin, /sbin, /lib*
- drop consolehelper to move /usr/sbin/* to /usr/bin
- alternatives symlinks?
- filesystem rpm, toplevel symlinks