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bug#33517: Problem booting when using btrfs subvolume for /gnu/store

From: Svante v. Erichsen
Subject: bug#33517: Problem booting when using btrfs subvolume for /gnu/store
Date: Sun, 27 Jan 2019 00:59:45 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

The subvolume and the mount point are independent issues.

For example, I have a subvolume named @ as root volume:

  (device "/dev/mapper/hd")
  (mount-point "/")
  (type "btrfs")
  (options "compress=lzo,ssd,subvol=@"))

It also appears that the subvolume name must be prepended to the path as if it
just was an ordinary directory (grub.cfg):

  linux /@/gnu/store/…

I could not yet confirm this from documentation, but it works like that for me.
(I semi-manually edit the generated grub.cfg currently.)

So, additionally to stripping the mount point, the subvolume needs to be
prepended (grub.scm):

  (let ((kernel (prepend-subvol device-subvol
                                (strip-mount-point device-mount-point
        (initrd (prepend-subvol device-subvol
                                (strip-mount-point device-mount-point

This would mean that the menu-entry structure needs the subvol information.
However, I wouldn't want to parse this from the options field in the file-system
entry, so I'd propose allowing a list there, maybe like this:

  (device "/dev/mapper/hd")
  (mount-point "/")
  (type "btrfs")
  (options '(("compress" . "lzo")
             ("subvol" . "@"))))

On the other hand, it might be surprising that declaring the options like this
would work for subvols, while using a string doesn't, especially when some older
documentation/blogs/gists is still hanging around on the internet, so maybe it
would be necessary to parse the options anyway (to this list structure).

I'm lacking experience in guix and guile, so making this work and submit a patch
will take me some time.

Do you think this is sensible?

Svante von Erichsen

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