Now that we have a fresh installed host (either as default or custom), we can better explain the OStree concepts and see them 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.
Commit ID
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 -v option.
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
OSname
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.
Refspec
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.
Deployments
We’ve used so far rpm-ostree. The same information can be obtained running an ostree command:
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