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OSTree is in many ways very evolutionary. It builds on concepts and ideas introduced from many different projects such as Systemd Stateless, Systemd Bootloader Spec, Chromium Autoupdate, the much older Fedora/Red Hat Stateless Project, Linux VServer and many more.
As mentioned elsewhere, OSTree is strongly influenced by package manager designs as well. This page is not intended to be an exhaustive list of such projects, but we will try to keep it up to date, and relatively agnostic.
Broadly speaking, projects in this area fall into two camps; either a tool to snapshot systems on the client side (dpkg/rpm + BTRFS/LVM), or a tool to compose on a server and replicate (ChromiumOS, Clear Linux). OSTree is flexible enough to do both.
Combining dpkg/rpm + (BTRFS/LVM)
In this approach, one uses a block/filesystem snapshot tool underneath the system package manager.
The oVirt Node imgbased tool is an example of this approach, as are a few others below.
Regarding BTRFS in particular - the OSTree author believes that Linux storage is a wide world, and while BTRFS is quite good, it is not everywhere now, nor will it be in the near future. There are other recently developed filesystems like f2fs, and Red Hat Enterprise Linux still defaults to XFS.
Using a snapshot tool underneath a package manager does help significantly. In the rest of this text, we will use "BTRFS" as a mostly generic tool for filesystem snapshots.
The obvious thing to do is layer BTRFS under dpkg/rpm, and have a
separate subvolume for /home so rollbacks don't lose your data. See
e.g. Fedora BTRFS Rollback Feature.
More generally, if you want to use BTRFS to roll back changes made by dpkg/rpm, you have to carefully set up the partition layout so that the files laid out by dpkg/rpm are installed in a subvolume to snapshot.
This problem in many ways is addressed by the changes OSTree forces,
such as putting all local state in /var (e.g. /usr/local ->
/var/usrlocal). Then one can BTRFS snapshot /usr. This gets pretty
far, except handling /etc is messy. This is something OSTree does
well.
In general, if one really tries to flesh out the BTRFS approach, a nontrivial middle layer of code between dpkg/rpm and BTRFS (or deep awareness of BTRFS in dpkg/rpm itself) will be required.
The OSTree author believes that having total freedom at the block
storage layer is better for general purpose operating systems. For
example, with OSTree, one is free to use BTRFS in any way you like -
you can use a subvolume for /home, or you can not.
Furthermore, in its most basic incarnation, the rpm/dpkg + BTRFS doesn't solve the race conditions that happen when unpacking packages into the live system, such as deleting the files underneath Firefox while it's running. One could unpack packages into a separate root, and switch to that, which gets closer to the OSTree architecture.
Note though OSTree does take advantage of BTRFS if installed on top of
it! In particular, it will use reflink for the copies of /etc if
available.
All of the above also applies if one replaces "BTRFS" with "LVM snapshots" except for the reflinks.
Finally, see the next portion around ChromiumOS for why a hybrid but integrated package/image system improves on this.
ChromiumOS updater
Many people who look at OSTree are most interested in using it as an updater for embedded or fixed-purpose systems, similar to use cases from the ChromiumOS updater.
The ChromiumOS approach uses two partitions that are swapped via the bootloader. It has a very network-efficient update protocol, using a custom binary delta scheme between filesystem snapshots.
This model even allows for switching filesystem types in an update.
A major downside of this approach is that the OS size is doubled on disk always. In contrast, OSTree uses plain Unix hardlinks, which means it essentially only requires disk space proportional to the changed files, plus some small fixed overhead.
This means with OSTree, one can easily have more than two trees (deployments). Another example is that the system OSTree repository could also be used for application containers.
Finally, the author of OSTree believes that what one really wants for many cases is image replication with the ability to layer on some additional components (e.g. packages) - a hybrid model. This is what rpm-ostree is aiming to support.
Ubuntu Image Based Updates
See https://wiki.ubuntu.com/ImageBasedUpgrades. Very architecturally similar to ChromeOS, although more interesting is discussion for supporting package installation on top, similar to rpm-ostree package layering.
Clear Linux Software update
The Clear Linux Software update system is not very well documented. This mailing list post has some reverse-engineered design documentation.
Like OSTree static deltas, it also uses bsdiff for network efficiency.
More information will be filled in here over time. The OSTree author believes that at the moment, the "CL updater" is not truly atomic in the sense that because it applies updates live, there is a window where the OS root may be inconsistent.
OLPC update
OSTree is basically a generalization of olpc-update, except using plain HTTP instead of rsync. OSTree has the notion of separate trees that one can track independently or parallel install, while still sharing storage via the hardlinked repository, whereas olpc-update uses version numbers for a single OS.
OSTree has built-in plain old HTTP replication which can be served
from a static webserver, whereas olpc-update uses rsync (more server
load, but more efficient on the network side). The OSTree solution to
improving network bandwidth consumption is via static deltas.
See this comment for a comparison.
NixOS
See NixOS. It was a very influential project for OSTree. NixOS and OSTree both support the idea of independent "roots" that are bootable.
In NixOS, the entire system is based on checksums of package inputs (build dependencies) - see Nix store. A both positive and negative of the Nix model is that a change in the build dependencies (e.g. being built with a newer gcc), requires a cascading rebuild of everything.
In OSTree, the checksums are of object content (including extended attributes). This means that any data that's identical is transparently, automatically shared on disk. It's possible to ask the Nix store to deduplicate, (via hard links and immutable bit), but this is significantly less efficient than the OSTree approach. The Nix use of the ext immutable bit is racy, since it has to be briefly removed to make a hard link.
At the lowest level, OSTree is just "git for binaries" - it isn't tied strongly to any particular build system. You can put whatever data you want inside an OSTree repository, built however you like. So for example, while one could make a build system that did the "purely functional" approach of Nix, it also works to have a build system that just rebuilds individual components (packages) as they change, without forcing a rebuild of their dependencies.
The author of OSTree believes that while Nix has some good ideas, forcing a rebuild of everything for a security update to e.g. glibc is not practical at scale.
Solaris IPS
See Solaris IPS. Broadly, this is a similar design as to a combination of BTRFS+RPM/deb. There is a bootloader management system which combines with the snapshots. It's relatively well thought through - however, it is a client-side system assembly. If one wants to image servers and replicate reliably, that'd be a different system.
Conary
See Conary Updates and Rollbacks. If rpm/dpkg are like CVS, Conary is closer to Subversion. It's not bad, but e.g. its rollback model is rather ad-hoc and not atomic. It also is a fully client side system and doesn't have an image-like replication with deltas.
bmap
See bmap. A tool for optimized copying of disk images. Intended for offline use, so not directly comparable.