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Re: Curious unionfs-ro-mount for /gnu/store in installation-image

From: Chris Marusich
Subject: Re: Curious unionfs-ro-mount for /gnu/store in installation-image
Date: Mon, 30 Oct 2017 18:56:37 -0700
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Hi Hartmut!

Hartmut Goebel <address@hidden> writes:

> immediately after booting the installation image (build according to [1]
> and run according to [2]), the "mount" command spits out *two* unionfs
> mounted points:
> address@hidden ~# mount
> unionfs on / type fuse.unionfs (rw,…)
> unionfs on /gnu/store type fuse.unionfs (ro,…)
> The first one ("/") is easy to explain: It is created by
> gnu/build/linux-boot.scm (mount-root-file-system) if #:volatile-root is
> true.
> But I'm curious, where the second one comes from. I've been searching
> this for two or three hours now and did not find, where this is defined.

The second one is a read-only bind-mount of the store.  The fact that
this exists is mentioned in the manual in the section titled "File
Systems"; look there for the string "%immutable-store".  Do 'grep -r
%immutable-store .' from the Guix source root directory, and you will
find where it is used - in particular:

* gnu/system/file-systems.scm - here, the variable %immutable-store is
  is defined with parameters that cause this file system to be
  bind-mounted.  The mechanism by which the bind-mounting occurs is
  defined elsewhere, but this might be what you were looking for.

* gnu/system/install.scm- here, the %immutable-store is used to
  construct the definition of the installation image.

Hope that helps!  For the record, I was able to find this information
because I knew that this was a "bind mount" to begin with.  So I did a
'grep -ir bind.*mount' and a 'grep -ir mount.*bind' in the Guix source
root directory and followed the clues from there.  The mount output by
itself is cryptic if you don't already know about bind mounts.


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