vm: Estimate the disk size by default.

* gnu/build/vm.scm (estimated-partition-size): New procedure.
* gnu/system/vm.scm (expression->derivation-in-linux-vm):
Change #:disk-image-size default to 'guess.
[builder]: When DISK-IMAGE-SIZE is 'guess, use
'estimated-partition-size' and compute and estimate of the image size.
(qemu-image): Likewise.
* guix/build/store-copy.scm (file-size, closure-size): New procedures.
* guix/scripts/system.scm (%default-options): Change 'image-size' to
'guess.
* doc/guix.texi (Building the Installation Image): Remove '--image-size'
flag from example.
(Invoking guix system): Document the image size estimate.

1 files changed, 8 insertions, 4 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 729ec081be..d61a5b7514 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -7877,9 +7877,8 @@ that.
 The installation image described above was built using the @command{guix
 system} command, specifically:
 
-@c FIXME: 1G is too much; see <http://bugs.gnu.org/23077>.
 @example
-guix system disk-image --image-size=1G gnu/system/install.scm
+guix system disk-image gnu/system/install.scm
 @end example
 
 Have a look at @file{gnu/system/install.scm} in the source tree,
@@ -16187,8 +16186,9 @@ size of the image.
 @item vm-image
 @itemx disk-image
 Return a virtual machine or disk image of the operating system declared
-in @var{file} that stands alone.  Use the @option{--image-size} option
-to specify the size of the image.
+in @var{file} that stands alone.  By default, @command{guix system}
+estimates the size of the image needed to store the system, but you can
+use the @option{--image-size} option to specify a value.
 
 When using @code{vm-image}, the returned image is in qcow2 format, which
 the QEMU emulator can efficiently use. @xref{Running GuixSD in a VM},
@@ -16251,6 +16251,10 @@ of the given @var{size}.  @var{size} may be a number of bytes, or it may
 include a unit as a suffix (@pxref{Block size, size specifications,,
 coreutils, GNU Coreutils}).
 
+When this option is omitted, @command{guix system} computes an estimate
+of the image size as a function of the size of the system declared in
+@var{file}.
+
 @item --root=@var{file}
 @itemx -r @var{file}
 Make @var{file} a symlink to the result, and register it as a garbage