bug#24344: Store on separate partition

[Carlos Sánchez de La Lama]

>>> I am trying to move the store to a separate partition, but this is
>>> problematic as you need the store at boot time for the system
>>> utilities (and to mount partitions, actually).
>>>
>> I tried to do it recently but failed.  My understanding is that it's
>> impossible, but I may be wrong.
>
> It would be interesting to fix it.  In theory there’s no reason why it
> shouldn’t work (you need to mark the /gnu/store file system as
> ‘needed-for-boot?’.)
>
> If you or Carlos could try it and report details to address@hidden, we
> could start from there.

I got it more-or-less working. What I did is a plain 0.11 install, but
before the "herd start cow-store /mnt" step I had prepared the target
filesystems so I had:

/dev/sda1 mount as /mnt
/dev/sdb1 mounted as /mnt/gnu

To make the store go into /dev/sdb1

My system definition included:

--8<---------------cut here---------------start------------->8---
  (file-systems (cons* (file-system
                         (device "my-root")
                         (title 'label)
                         (mount-point "/")
                         (type "ext4"))
                       (file-system
                         (device "/dev/sdb1")
                         (mount-point "/gnu")
                         (type "ext4")
                         (needed-for-boot? #t))
                       %base-file-systems))
--8<---------------cut here---------------end--------------->8---

When rebooting the installed system, I have to edit GRUB command line
(pressing 'e'), which was:

--8<---------------cut here---------------start------------->8---
  search --file --set 
/gnu/store/2qh58nr4sxc5jwv2ng9sqq06niab8sjp-linux-libre-4.7/bzImage
  linux /gnu/store/2qh58nr4sxc5jwv2ng9sqq06niab8sjp-linux-libre-4.7/bzImage 
--root=my-root --system=/gnu/store/4ji3d5v5ci91l69fgi4hdi73kc558whp-system 
--load=/gnu/store/4ji3d5v5ci91l69fgi4hdi73kc558whp-system/boot
  initrd /gnu/store/4ji3d5v5ci91l69fgi4hdi73kc558whp-system/initrd
--8<---------------cut here---------------end--------------->8---

By:

1) removing "/gnu" from search command, so GRUB root (not kernel root)
   is correctly set as (hd1,msdos1), i.e. /dev/sdb1
2) removing "/gnu" from linux command, so the kernel is correctly loaded
   by GRUB (/gnu is not removed from --system or --load, as those
   commands are interpreted by gnu/build/linux-boot.scm after all
   needed-for-boot filesystems have been mounted in their correct mount
   points).
3) changing initrd path. Removing "/gnu" is not enough, as the path in
   grub.cfg is a symlink to the real initrd, pointing into /gnu/store
   which is not available at initrd-loading time. So I placed the
   synlink destination there, removing "/gnu", so:

initrd /store/<hash>-base-initrd/initrd

And voilá, I can boot into my newly installed GuixSD with /gnu/store in
a separate partition.

Some other files (modules, background image) fail to be accesses by GRUB
as well with generated grub.cfg, but those are not essential to make the
system boot, in my system at least.

BR

Carlos