rpm-ostree/doc/background.md

Package systems versus image systems

Broadly speaking, software update systems for operating systems tend to fall cleanly into one of two camps: package-based or image-based.

Package system benefits and drawbacks

• • Highly dynamic, fast access to wide array of software
• • State management in /etc and /var is well understood
• • Can swap between major/minor system states (apt-get upgrade is similar to apt-get dist-upgrade)
• • Generally supports any filesystem or partition layout
• • As package set grows, testing becomes combinatorially more expensive
• • Live system mutation, no rollbacks

Image benefits and drawbacks

• • Ensures all users are running a known state
• • Rollback supported
• • Can achieve efficient security via things like dm-verity
• • Many image systems have a read-only /etc, and writable partitions elsewhere
• • Must reboot for updates
• • Usually operate at block level, so require fixed partition layout and filesystem
• • Many use a "dual root" mode which wastes space and is inflexible
• • Often paired with a separate application mechanism, but misses out on things that aren't apps
• • Administrators still need to know content inside

How RPM-OSTree provides a middle ground

rpm-ostree in its default mode feels more like image replication, but the underlying architecture allows a lot of package-like flexibility.

In this default mode, packages are composed on a server, and clients can replicate that state reliably. For example, if one adds a package on the compose server, clients get it. If one removes a package, it's also removed when clients upgrade.

One simple mental model for rpm-ostree is: imagine taking a set of packages on the server side, install them to a chroot, then doing git commit on the result. And imagine clients just git pull -r from that. What OSTree adds to this picture is support for file uid/gid, extended attributes, handling of bootloader configuration, and merges of /etc.

To emphasize, replication is at a filesystem level - that means things like SELinux labels and uid/gid mappings are assigned on the server side.

On the other hand, rpm-ostree works on top of any Unix filesystem. It will not interfere with any filesystem or block-level snapshots or backups such as LVM or BTRFS.

Who should use this?

Currently, rpm operates on a read-only mode on installed systems; it is not possible to add or remove anything on the client. If this matches your deployment scenario, rpm-ostree is a good choice. Classic examples of this are fixed purpose server farms, "corporate standard build" laptop/desktops, and embedded devices.

Of course, one can pair it with a dynamic application mechanism such as Docker, and have a reliable base, with a flexible application tool. This is the rationale behind Project Atomic.

Is it worth supporting composes both on client and server?

In short, our belief is yes. Long term, rpm-ostree offers a potential unified tooling via package layering.