I run 'guix system vm-image' VMs on a GuixSD server (hostname g1). The
VMs are configured to get substitutes from g1 in addition to the
official Guix servers.

This works great except that the VMs are failing to find/use some of the
substitutes that are available on g1.  E.g., in the attached log
(setup.log.gz) of the first 'guix package' issued on VM sysi58, curl is
built ...

building /gnu/store/24ag580271wa640529ycykdwj0lk0g6z-curl-7.61.1.tar.xz.drv...
downloading from https://curl.haxx.se/download/curl-7.61.1.tar.xz...
building /gnu/store/17lw3svpjqygpj739yynyz6b8abddikx-curl-7.61.1.drv...

... when the substitute is available in the g1 store ...

  /gnu/store:
  -r--r--r--    2 root root            3747 Dec 31  1969
   17lw3svpjqygpj739yynyz6b8abddikx-curl-7.61.1.drv

This is "infrequent", in the sense that a high percentabe of the
substitutes available from g1 are being found and used. However, these
other packages for which substitutes are available on g1 are also built:

downloading from ftp://ftp.knackered.org/pub/psutils/psutils.tar.gz...
downloading from http://download.osgeo.org/libtiff/tiff-4.0.9.tar.gz...
downloading from https://ftpmirror.gnu.org/gnu/groff/groff-1.22.3.tar.gz...
downloading from https://github.com/apple/cups/releases/download/v2.2.8/cups-2.2.8-source.tar.gz...
downloading from https://github.com/ArtifexSoftware/ghostpdl-downloads/releases/download/gs924/ghostscript-9.24.tar.xz...
downloading from https://gnupg.org/ftp/gcrypt/libgcrypt/libgcrypt-1.8.3.tar.bz2...
downloading from http://www.x.org/releases/individual/lib/libX11-1.6.6.tar.bz2...
downloading from http://www.x.org/releases/individual/lib/xtrans-1.3.5.tar.bz2...

Details:

Using Guix from Git:

Server g1: sys.scm    v0.15.0-2913-g21c51ebd6

VM sysi58: sysi58.scm v0.15.0-3115-g7571ec357

TIA - George