Concepts in Action
Querying the deployed filetrees
The first thing to do is to run a command that tells us what is installed on the machine and when. Since it’s a fresh install from the CD, there is only one bootable filetree image deployed.
root@photon-7c2d910d79e9 [ ~ ]# rpm-ostree status State: idle Deployments: ● ostree://photon:photon/4.0/x86_64/minimal Version: 4.0_minimal (2021-02-20T07:15:43Z) Commit: 965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f
Bootable filetree version
4.0_minimal is not the Linux Photon OS release version, nor daily build, but rather a human readable, self-incrementing version associated with every commit that brings file/package updates. Think of this as version 0. The following versions are going to be 4.0_minimal.1, 4.0_minimal.2, 4.0_minimal.3 and so on.
The ID listed is actually the first 5 bytes (10 hex digits) of the commit hash. If you want to see the verbose mode, use the
root@photon-7c2d910d79e9 [ ~ ]# rpm-ostree status -v State: idle AutomaticUpdates: disabled Deployments: ● ostree://photon:photon/4.0/x86_64/minimal Version: 4.0_minimal (2021-02-20T07:15:43Z) Commit: 965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f └─ photon (2021-02-20T07:11:24Z) Staged: no StateRoot: photon
RPM OStree Options
To see the list of options available with the rpm-ostree command, use the -h option.
root@photon-7c2d910d79e9 [ ~ ]# rpm-ostree -h Usage: rpm-ostree [OPTION…] COMMAND Builtin Commands: compose Commands to compose a tree cleanup Clear cached/pending data db Commands to query the RPM database deploy Deploy a specific commit rebase Switch to a different tree rollback Revert to the previously booted tree status Get the version of the booted system upgrade Perform a system upgrade reload Reload configuration usroverlay Apply a transient overlayfs to /usr cancel Cancel an active transaction initramfsEnable or disable local initramfs regeneration install Overlay additional packages uninstallRemove overlayed additional packages override Manage base package overrides resetRemove all mutations refresh-md Generate rpm repo metadata kargsQuery or modify kernel arguments Help Options: -h, --help Show help options Application Options: --versionPrint version information and exit
The OS Name identifies the operating system installed. All bootable filetrees for the same OS will share the /var directory, in other words applications installed in one booted image into this directory will be available in all other images.
If a new set of images are created for a different OS, they will receive a fresh copy of /var that is not shared with the previous OS images for the initial OS. In other words, if a machine is dual boot for different operating systems, they will not share each other’s /var content, however they will still merge 3-way /etc.
The Refspec is a branch inside the repo, expressed in a hierarchical way. In this case, it’s the default branch that will receive package updates for the Photon OS 1.0 Minimal installation profile on Intel platforms. There could be other branches in the future, for example photon/3.0/x86_64/full that will match the Full installation profile (full set of packages installed).
Think of Refspec as the head of the minimal branch (just like in git) at the origin repo. On the replicated, local repo at the host, minimal is a file that contains the latest commit ID known for that branch.
root@photon-7c2d910d79e9 [ ~ ]# cat /ostree/repo/refs/remotes/photon/photon/4.0/x86_64/minimal 820b584a6f90bf6b9b8cb6aad8c093064b88d0ab686be8130baa03d68917ad88
Why are there two ‘photon’ directory levels in the remotes path? The photon: prefix in the Refspec listed by
rpm-ostree status corresponds to the first photon directory in the remotes path and is actually the name given to the remote that the host is connected to, which points to an http or https URL. We’ll talk about remotes later, but for now think of it as a namespace qualifier. The second photon is part of the Refspec path itself.
We’ve used so far
rpm-ostree. The same information can be obtained running an
root@photon-7c2d910d79e9 [ ~ ]# ostree admin status * photon 965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f.0 Version: 4.0_minimal origin refspec: photon:photon/4.0/x86_64/minimal
But where is this information stored? As you may have guessed, the local repo stores the heads of the deployed trees - the most recent commitment ID, just like Git does:
root@photon-7c2d910d79e9 [ ~ ]# cat /ostree/repo/refs/heads/ostree/0/1/0 965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f
This also where this command that lists the references (local heads and remotes) takes its data from:
root@photon-7c2d910d79e9 [ ~ ]# ostree refs photon:photon/4.0/x86_64/minimal ostree/0/1/0
Based on that, it could find the root of the deployment that it boots from. The actual filetree is deployed right here:
root@photon-7c2d910d79e9 [ ~ ]# ls -l /ostree/deploy/photon/deploy/965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f.0 total 36 lrwxrwxrwx 2 root root7 Feb 23 05:43 bin -> usr/bin drwxr-xr-x 2 root root 4096 Jan 1 1970 boot drwxr-xr-x 2 root root 4096 Jan 1 1970 dev drwxr-xr-x 45 root root 4096 Feb 23 06:08 etc lrwxrwxrwx 2 root root8 Feb 23 05:43 home -> var/home lrwxrwxrwx 3 root root7 Feb 23 05:43 lib -> usr/lib lrwxrwxrwx 3 root root7 Feb 23 05:43 lib64 -> usr/lib lrwxrwxrwx 2 root root9 Feb 23 05:43 media -> run/media lrwxrwxrwx 2 root root7 Feb 23 05:43 mnt -> var/mnt lrwxrwxrwx 2 root root7 Feb 23 05:43 opt -> var/opt lrwxrwxrwx 2 root root 14 Feb 23 05:43 ostree -> sysroot/ostree drwxr-xr-x 2 root root 4096 Jan 1 1970 proc lrwxrwxrwx 2 root root 12 Feb 23 05:43 root -> var/roothome drwxr-xr-x 2 root root 4096 Jan 1 1970 run lrwxrwxrwx 2 root root8 Feb 23 05:43 sbin -> usr/sbin lrwxrwxrwx 2 root root7 Feb 23 05:43 srv -> var/srv drwxr-xr-x 2 root root 4096 Jan 1 1970 sys drwxr-xr-x 4 root root 4096 Feb 23 05:44 sysroot lrwxrwxrwx 2 root root 11 Feb 23 05:43 tmp -> sysroot/tmp drwxr-xr-x 10 root root 4096 Jan 1 1970 usr drwxr-xr-x 8 root root 4096 Feb 23 05:44 var
So how is a deployment linked to a specific branch, originating from a remote repo? Well, there is a file next to the deployed filetree root directory with the same name and .origin suffix, that contains exactly this info:
root@photon-7c2d910d79e9 [ ~ ]# cat /ostree/deploy/photon/deploy/965c1abeb048e1a8ff77e9cd34ffccc5e3356176cda3332b4ff0e7a6c66b661f.0.origin [origin] refspec=photon:photon/4.0/x86_64/minimal
Fast forwarding a bit, if there is a new deployment due to an upgrade or rebase, a new filetree will be added at the same level, and a new .origin file will tie it to the remote branch it originated from.
The photon directory in the path is the actual OSname. Multiple deployments of same OS will share a writable /var folder.
root@photon-7c2d910d79e9 [ ~ ]# ls -l /ostree/deploy/photon/var/ total 52 drwxr-xr-x 5 root root 4096 Feb 23 05:44 cache drwxr-xr-x 2 root root 4096 Feb 23 05:44 home drwxr-xr-x. 17 root root 4096 Feb 23 05:44 lib drwxr-xr-x 2 root root 4096 Feb 23 05:44 local lrwxrwxrwx. 1 root root 11 Feb 23 05:44 lock -> ../run/lock drwxr-xr-x. 4 root root 4096 Feb 23 05:44 log drwxr-xr-x 2 root root 4096 Feb 23 05:44 mail drwxr-xr-x 2 root root 4096 Feb 23 05:44 mnt drwxr-xr-x 4 root root 4096 Feb 23 05:44 opt drwx------ 4 root root 4096 Feb 23 06:09 roothome lrwxrwxrwx. 1 root root6 Feb 23 05:44 run -> ../run drwxr-xr-x 3 root root 4096 Feb 23 05:44 spool drwxr-xr-x 2 root root 4096 Feb 23 05:44 srv drwxrwxrwt. 5 root root 4096 Feb 23 06:08 tmp drwxr-xr-x 11 root root 4096 Feb 23 05:44 usrlocal