My PR for the singularity "underlay" feature is now submitted:
https://github.com/singularityware/singularity/pull/1638
I have been told by the singularity core development team that they
might not have the resources to adequately review this for a 2.x release
and might have to wait until it is rewritten for 3.0. If you believe
this is important to get into the 2.6 release, please comment in the PR
your experiences with testing it, and carefully review the code as much
as you can, so the singularity core team can feel confident enough in it
without having to put in a lot of their resources for review. 2.6 is
likely to be the last 2.N (feature release) before 3.0, and this is
definitely more than a bug fix so it can't go into a 2.6.X.
It would also be helpful to make comments in the ticket about how strong
your desire is that this get into the 2.6 release.
Worst case, if they reject it or put it off for too long, we have the
option of adding it as a patch to the OSG & EPEL singularity-2.6
release.
In order to make it easier for you, I did an OSG scratch build including
this PR and everything else currently in the development-2.x branch and
you can find the 2.5.1-3 rpms here:
EL7:
https://koji.chtc.wisc.edu/koji/taskinfo?taskID=273473
EL6:
https://koji.chtc.wisc.edu/koji/taskinfo?taskID=273472
underlay is enabled by default in /etc/singularity/singularity.conf if
overlay does not work. On EL7 you can disable overlay by setting the
environment variable SINGULARITY_DISABLE_OVERLAYFS=1, by setting enable
overlay = no in singularity.conf, or by using the exec "-u" option to
run unprivileged (assuming you have enabled unprivileged user namespaces).