Discussion among Rob, Xin and Wei:

It is better to de-couple the CentOS 7 migration and Singularity deployment, so that C7 migration can happen sooner. ADC doc for C7 migration:  

https://twiki.cern.ch/twiki/bin/view/AtlasComputing/CentOS7Readiness

US sites has the options to take Rolling Transition or Big Bang Transition. We will work site-by-site to help the transition. ADC strongly suggest the Singularity 2.4.2 rpms be installed on C7 WNs.

• AGLT2 is in the process, using Rolling Transition
• BNL is doing Rolling Transition, plus Containerized WNs (see below). So moving forward unnoticed.

On Singularity: see presentation at ADC Site Jamboree: 

1. Singularity version 2.4.2, no 2.3.x
2. Ultimately pilot 2 will evoke Singularity - compatible with US APF (and EU APF and aCT)
   • Pilot 2 is not quite ready.
3. Incompatible with Containerized WNs (Encapsulate payload in a Container)
   • Containerized WNs is not a requirement, and you are on your own to support it
   • But not forbidden either (good for learning and trying it out).  
4. Will need container_type and container_options setting in AGIS / Panda Queue
5. For HPCs, investigating methods to reduce container image size
   • single release image
   • use SquashFS instead of Ext3 - doing this for NERSC - reduce by a factor of 3