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rpm-ostree/docs/manual/background.md
Doug Hellmann d160ed181c docs/background: Fix benefits and drawbacks formatting 
The lists of pros and cons for each approach in the background section
of the manual use markdown formatting that is rendered as nested
lists. Separate the lists of benefits and drawbacks so that readers of
https://rpm-ostree.readthedocs.io/en/latest/manual/background/ and the
rendered markdown on github can understand which items are pros and
which are cons.

Signed-off-by: Doug Hellmann <dhellmann@redhat.com>
2020-05-15 12:58:17 -07:00

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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

Benefits

  • 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

Drawbacks

  • As package set grows, testing becomes combinatorially more expensive
  • Live system mutation, no rollbacks

Image benefits and drawbacks

Benefits

  • Ensures all users are running a known state
  • Rollback supported
  • Easier to verify system integrity

Drawbacks

  • 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.