Anatomy of an OSTree repository

Core object types and data model

OSTree is deeply inspired by git; the core layer is a userspace content-addressed versioning filesystem. It is worth taking some time to familiarize yourself with Git Internals, as this section will assume some knowledge of how git works.

Its object types are similar to git; it has commit objects and content objects. Git has "tree" objects, whereas OSTree splits them into "dirtree" and "dirmeta" objects. But unlike git, OSTree's checksums are SHA256. And most crucially, its content objects include uid, gid, and extended attributes (but still no timestamps).

Commit objects

A commit object contains metadata such as a timestamp, a log message, and most importantly, a reference to a dirtree/dirmeta pair of checksums which describe the root directory of the filesystem. Also like git, each commit in OSTree can have a parent. It is designed to store a history of your binary builds, just like git stores a history of source control. However, OSTree also makes it easy to delete data, under the assumption that you can regenerate it from source code.

Dirtree objects

A dirtree contains a sorted array of (filename, checksum) pairs for content objects, and a second sorted array of (filename, dirtree checksum, dirmeta checksum), which are subdirectories.

Dirmeta objects

In git, tree objects contain the metadata such as permissions for their children. But OSTree splits this into a separate object to avoid duplicating extended attribute listings.

Content objects

Unlike the first three object types which are metadata, designed to be mmap()ed, the content object has a separate internal header and payload sections. The header contains uid, gid, mode, and symbolic link target (for symlinks), as well as extended attributes. After the header, for regular files, the content follows.

The OSTree data format intentionally does not contain timestamps. The reasoning is that data files may be downloaded at different times, and by different build systems, and so will have different timestamps but identical physical content. These files may be large, so most users would like them to be shared, both in the repository and between the repository and deployments.

This could cause problems with programs that check if files are out-of-date by comparing timestamps. For Git, the logical choice is to not mess with timestamps, because unnecessary rebuilding is better than a broken tree. However, OSTree has to hardlink files to check them out, and commits are assumed to be internally consistent with no build steps needed. For this reason, OSTree acts as though all timestamps are set to time_t 0, so that comparisons will be considered up-to-date. Note that for a few releases, OSTree used 1 to fix warnings such as GNU Tar emitting "implausibly old time stamp" with 0; however, until we have a mechanism to transition cleanly to 1, for compatibilty OSTree is reverted to use zero again.

Repository types and locations

Also unlike git, an OSTree repository can be in one of four separate modes: bare, bare-user, bare-user-only, and archive. A bare repository is one where content files are just stored as regular files; it's designed to be the source of a "hardlink farm", where each operating system checkout is merely links into it. If you want to store files owned by e.g. root in this mode, you must run OSTree as root.

The bare-user is a later addition that is like bare in that files are unpacked, but it can (and should generally) be created as non-root. In this mode, extended metadata such as owner uid, gid, and extended attributes are stored but not actually applied. The bare-user mode is useful for build systems that run as non-root but want to generate root-owned content, as well as non-root container systems.

There is a variant to the bare-user mode called bare-user-only. Unlike bare-user, neither ownership nor extended attributes are stored. These repos are meant to to be checked out in user mode (with the -U flag), where this information is not applied anyway. The main advantage of bare-user-only is that repos can be stored on filesystems which do not support extended attributes, such as tmpfs.

In contrast, the archive mode is designed for serving via plain HTTP. Like tar files, it can be read/written by non-root users.

On an OSTree-deployed system, the "system repository" is /ostree/repo. It can be read by any uid, but only written by root. Unless the --repo argument is given to the ostree command, it will operate on the system repository.

Refs

Like git, OSTree uses the terminology "references" (abbreviated "refs") which are text files that name (refer to) to particular commits. See the Git Documentation for information on how git uses them. Unlike git though, it doesn't usually make sense to have a "master" branch. There is a convention for references in OSTree that looks like this: exampleos/buildmaster/x86_64-runtime and exampleos/buildmaster/x86_64-devel-debug. These two refs point to two different generated filesystem trees. In this example, the "runtime" tree contains just enough to run a basic system, and "devel-debug" contains all of the developer tools and debuginfo.

The ostree supports a simple syntax using the caret ^ to refer to the parent of a given commit. For example, exampleos/buildmaster/x86_64-runtime^ refers to the previous build, and exampleos/buildmaster/x86_64-runtime^^ refers to the one before that.

The summary file

A later addition to OSTree is the concept of a "summary" file, created via the ostree summary -u command. This was introduced for a few reasons. A primary use case is to be compatible with Metalink, which requires a single file with a known checksum as a target.

The summary file primarily contains two mappings:

  • A mapping of the refs and their checksums, equivalent to fetching the ref file individually
  • A list of all static deltas, along with their metadata checksums

This currently means that it grows linearly with both items. On the other hand, using the summary file, a client can enumerate branches.

Further, fetching the summary file over e.g. pinned TLS creates a strong end-to-end verification of the commit or static delta.

The summary file can also be GPG signed (detached). This is currently the only way to provide GPG signatures (transitively) on deltas.

If a repository administrator creates a summary file, they must thereafter run ostree summary -u to update it whenever a ref is updated or a static delta is generated.