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rpm-ostree/docs/manual/background.md
Colin Walters 80fd2e883b README.md: Add a "why" section, update background.md 
Point at FCOS and not Project Atomic.  Add an inline "Why"
section since people will want to know that right away.

(An great thing about Github is the prevalence it gives to `README.md`;
 projects should use that as an "elevator pitch")

Drop outdated bits in `background.md`.

Closes: #1895
Approved by: jlebon
2019-09-04 20:09:13 +00:00

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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
    • 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 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.