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Re: Remounting tmpfs

From: rendaw
Subject: Re: Remounting tmpfs
Date: Thu, 18 Apr 2019 05:50:17 +0900
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.3

On 4/18/19 5:31 AM, Ludovic Courtès wrote:
> Hi,
> address@hidden skribis:
>> On 4/9/19 11:58 PM, Ludovic Courtès wrote:
>>> Hello,
>>> address@hidden skribis:
>>>> On a system I'm porting to guix I have 2GB tmpfs with subdirectories
>>>> like /tmpfs/etc that I remount to /etc with an overlay filesystem.
>>>> The current way I do this in systemd is making a service dependency
>>>> between the /tmpfs and /etc mounts that mkdirs /tmpfs/etc and
>>>> /tmpfs/etc_work, but AFAICT filesystem definitions in guix can only have
>>>> filesystem dependencies.
>>>> Are there any other ways I can do this without copying/pasting/modifying
>>>> gobs of core guix code into my system definition?  Like somehow
>>>> appending (mkdir /tmpfs/etc) onto the tmpfs filesystem service start
>>>> procedure or something.
>>> In Guix /etc is mostly populated by “activation programs”, which are
>>> generated from your config.  So I’m not sure what you describe would
>>> make much sense.
>> So if /etc can be read-only and boot I'm probably fine... my experience
>> with other distros was that some other processes needed to write to it. 
>> Ex: modifying resolv.conf.
> /etc is writable because of things like ‘resolv.conf’.
> /etc consists mostly of immutable files derived directly from your OS
> config (/etc/passwd, /etc/hosts, /etc/polkit-1, /etc/pam.d, etc.), along
> with files that contains bits of state (/etc/shadow, /etc/resolv.conf.)
> The former are directly managed by Guix, while the latter are either
> left as is or touched with care by Guix (/etc/shadow in particular.)
>>> Now, you could try to add a file system declaration that mounts /etc,
>>> with (needed-for-boot? #t).
>> My goal is to have a read-only / mount with the ability for programs to
>> make temporary modifications for operational purposes when necessary, in
>> limited scopes (like /etc).  Can you elaborate on what you're suggesting
>> here?  Mounting something other than the overlayfs on /etc would hide
>> the system config files.  I might be able to use another mount to create
>> a pseudo- /tmpfs/etc_work subdirectory but it sounds kind of wormy and
>> overlayfs requires the upper dir and workdir to be the same filesystem
>> which I think precludes doing any mounting for those subdirectories.
> The overlay makes a lot of sense.  This is what ‘guix system vm’ does:
> see the #:volatile-root? parameter of ‘raw-initrd’.
> Perhaps you could simply set #:volatile-root? #t in your initrd to
> obtain what you want?
> Thanks,
> Ludo’.

Thanks, yeah, that sounds exactly like what I want!  TBH I think
something's going on strange with my threads, perhaps because I messed
up the replies -- after much source reading I found volatile-root and
asked about it in my disk-image thread.  TBH I'm not clear how that
would be set in the config to use with disk-image to get a whole system.

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