Comment 2 for bug 1879779

This affects me as well - trying to do an autoinstall with mounting btrfs / with compression enabled before /target is written.

From what I can tell Subiquity does not parse storage:config:mount:options in filesystem.py:add_filesystem and there is no place for it to be stored in the filesytem model for it to be passed to curtin. The options are silently dropped.

This should be noted in the documentation.

https://ubuntu.com/server/docs/install/autoinstall-reference

```
action-based config
For full flexibility, the installer allows storage configuration to be done using a syntax which is a superset of that supported by curtin, described at https://curtin.readthedocs.io/en/latest/topics/storage.html.
```

It is not a superset if mount:options are not used.