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docs: Add more details about 3-way merge

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This came up a few times so let's go into more details in the docs.
This commit is contained in:
Jonathan Lebon 2021-04-30 10:50:15 -04:00
parent 89481ca4be
commit 788e171b7c
1 changed files with 20 additions and 4 deletions
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docs/atomic-upgrades.md
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@ -69,10 +69,26 @@ given commit is deployed more than once, it will be incremented.
This is supported because the previous deployment may have
configuration in `/etc` that we do not want to use or overwrite.
Now that we have a deployment directory, a 3-way merge is
performed between the (by default) currently booted deployment's
`/etc`, its default
configuration, and the new deployment (based on its `/usr/etc`).
Now that we have a deployment directory, a 3-way merge is performed
between the (by default) currently booted deployment's `/etc`, its
default configuration, and the new deployment (based on its `/usr/etc`).
How it works is:
- Files in the currently booted deployment's `/etc` which were modified
from the default `/usr/etc` (of the same deployment) are retained.
- Files in the currently booted deployment's `/etc` which were not
modified from the default `/usr/etc` (of the same deployment) are
upgraded to the new defaults from the new deployment's `/etc/etc`.
Roughly, this means that as soon as you modify or add a file in `/etc`,
this file will be propagated forever as is (though there is a
corner-case, where if your modification eventually exactly matches a
future default file, then the file will go back to following future
default updates from that point on).
You can use `ostree admin config-diff` to see the differences between
your booted deployment's `/etc` and the OSTree defaults. A command like
`diff {/usr,}/etc` will additional print line-level differences.
## Atomically swapping boot configuration