Deprecate 'file' storage driver in R4.1, remove in R5.0

[marmarek]

Qubes OS version (if applicable)

R4.1

Affected component(s) or functionality (if applicable)

core

Brief summary

1. Add some verbose notification about using deprecated storage driver in R4.1. I think this can be a notification at the system startup (can be added to the code of one of the widgets, for example qui-updates one). Also, add a warning when creating new "file" pool. Maybe also when listing storage pools?
2. Do not create "file" pool automatically during system installation. But do create file-reflink for /var/lib/qubes directory, for compatible filesystems.
3. Document migration off the file driver (either to file-reflink or to lvm), and link to it from release notes and the notification. In-place migration is preferred if possible, otherwise document reinstall method.

Additional context

The file storage driver is very inefficient and also some features are impossible to achieve using its old architecture (it simulates CoW volumes using dm-snapshot, which limits snapshots levels to 1). Example issues caused by its inferior features:

• Corrupted backup/clone of running VMs that are stored in a 'file' driver pool #4324
• Template cloning in R4.1 is significantly slower than in R4.0 #6329
• VMs in file-based storage pools cannot use fstrim for /rw #3255

[rustybird]

3. Document migration off the file driver (either to file-reflink or to lvm), and link to it from release notes and the notification. In-place migration is preferred if possible, otherwise document reinstall method.

As a first stab at this, here's howto-qubes-btrfs-convert.md ("EXPERIMENTAL: Convert a Qubes R4.0 installation from file on ext4 to file-reflink on btrfs")

[Rudd-O]

Is there going to be an option for those of us using ZFS instead of LVM?

[DemiMarie]

@Rudd-O I suggest implementing a ZFS storage driver instead, or finishing the existing one. I would be willing to review a PR.

[rustybird]

Worth mentioning that the development of reflink support for ZFS seems to be making good progress. Maybe it'll land before R4.2.