Re: btrfs recommended layout for snapshots?

[Andrew Tropin <andrew@trop.in>]

[-- Attachment #1: Type: text/plain, Size: 4125 bytes --]

On 2023-08-18 01:58, Nicolas Graves wrote:

> On 2023-08-16 10:10, Nicolas Graves wrote:
>
>> I guess it's possible to do the same with my home as well (thus only
>> saving actual data and not consecutive linking metadata), but that might
>> require some more time and fine-grained applications considerations.
>>
>> One weakness from this impermanence feature is that it's actually
>> application-dependent. For guix-system it's not very damaging (except if
>> we want very low-level optimizations, like setting nodatacow on
>> subvolumes with databases etc), but for guix-home, it makes things much
>> more difficult. @Andrew Tropin : maybe that's something we could in RDE in
>> a state-btrfs in (gnu home-services state) if we find a way to migrate
>> directories to subvolumes safely and reproducibly.
>
> Some notes about more progress I've done.
>
> My attempt to also load the /home subvolume on tmfps has quite
> progressed. I've created the following subvolumes :
>
> ;; App related (apps who doesn't entirely follow the XDG base directory
> ;; specification and save data or cache outside of XDG_DATA_HOME,
> ;; XDG_STATE_HOME and XDG_CACHE_HOME. Other users may need other app dirs.
>
> /home/graves/.config/chromium
> /home/graves/.config/emacs
> /home/graves/.config/libreoffice
> /home/graves/.config/guix
> /home/graves/.ssh
>
> ;; XDG_CACHE_HOME, XDG_STATE_HOME, XDG_DATA_HOME (I'm using RDE)
>
> /home/graves/.cache
> /home/graves/.local
>
> ;; And some personal want-to-save directories.
>
> /home/graves/archives
> /home/graves/resources
> /home/graves/projects
> /home/graves/spheres
>
> The only thing that seems to get in my way to achieve this properly
> is... .guix-home! Which I don't want to backup since it's only a link
> and that would require at least /home/graves/ to be snapshotted.
>
> I thus have a proposition for discussion :
> Make .guix-home XDG base dir compliant by storing a symlink
> in $XDG_CONFIG_DIR/guix/home to /var/guix/per-user/$user/guix-home
> instead of the current default of the symlink
> in /home/$user/.guix-home to the actual object in the store.
>
> This was discussed in a previous mail thread :
> "RFC: Configurable placing of the ~/.guix-home symlink"
> With Andrew concluding that
>
>>  Back in the day, the implementation of Guix Home required a symlink in
>>  home directory, right now due to improvements in symlink-manager and
>>  reconfigure code it's probably not necessary and with a few patches
>>  /var/guix/profiles/per-user/bob/guix-home/ can be used instead.
>
> With a first glance, I think it's possible to do in the code, since the
> home-run-on-first-login-service-type already gets the UID of the user,
> and with the following guile function :
>
> Scheme Procedure: passwd:name pw
>     The name of the userid.
>
> we should be able to get the name of the user and replace
> ~/.guix-home with /var/guix/per-user/$user/guix-home everywhere.
> So the code where a hardlink is needed will be, and the "pleasing UX of
> searching within guix home" would also be possible.
>
> I also don't really see the reason why .guix-home shouldn't be
> $XDG_CONFIG_DIR/guix/home since it's really user-specific and unique
> (and XDG user dirs are too), unlike .guix-profile.

I don't have all the context loaded in my head right now, but it's
probably possible now, and we can try to implement it.  Feel free to
send a patch and Cc me or continue the discussion on "RFC: Configurable
placing of the ~/.guix-home symlink".

>
> This may be the one of the only missing step to make the (manual and
> only with directories (btrfs subvolumes), at least for now)
> implementation of impermanence (a quick reminder of the idea implemented
> by nix here : https://nixos.wiki/wiki/Impermanence) on with guix home, I
> would appreciate some feedback comments on the idea ;) (another step
> would be to actually activate the home environment on login in
> home-shell-profile-service-type, but migrating .guix-home would be a
> requirement).

-- 
Best regards,
Andrew Tropin

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]