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
/varis well understood
- Can swap between major/minor system states (
apt-get upgradeis similar to
- 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
- Easier to verify system integrity
- 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
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
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.