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docs: Expand on filesystem /var #395

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Mar 18, 2024
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docs: Expand on filesystem /var #395

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11 changes: 10 additions & 1 deletion docs/src/filesystem.md
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Expand Up @@ -35,7 +35,16 @@ mount points (whether network or `tmpfs`)

As of OSTree v2024.3, by default [content in /var acts like a Docker VOLUME /var](https://github.com/ostreedev/ostree/pull/3166/commits/f81b9fa1666c62a024d5ca0bbe876321f72529c7).

This means that the content from the container image is copied at *initial installation time*, and not updated thereafter.
This means that the content from the container image is copied at initial installation time, and *not updated thereafter*.

The rationale for this is to keep operating system upgrades from touching machine-local data by default.
If the system is rolled back to a previous bootloader entry, the `/var` content remains. This also
makes it possible to "stage" new operating system updates in an alternative root without affecting `/var` content.

A common case is for applications to want some directory structure (e.g. `/var/lib/postgresql`) to be pre-created.
It's recommended to use [systemd tmpfiles.d](https://www.freedesktop.org/software/systemd/man/latest/tmpfiles.d.html)
for this. An even better approach where applicable is [StateDirectory=](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#RuntimeDirectory=)
in units.

## Other directories

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