Some success in testing sites with no local production data storage for ATLAS VO.

For a while we have had a few  of the smaller sites (T3s) in the UK running for ATLAS with out any storage at the site.We recently tried to run Birmingham as a completely diskless site with it using storage at Manchester.  This was mostly successful; the saturation of the WAN connection at Manchester which was always considered a worrying possibility was seen. This has helped inform ATLAS's opinions on how to implement diskless sites which was then presented at the ATLAS Site Jamboree this month.

We intend to try using XCache at Birmingham instead to see that is an alternative approach which might succeed. WE are also looking into using the ARC Control tower to pre-place data for ARC-CEs. Main issue is how this conflicts with VO wish for last minute payload changes within pilot jobs.

I would also just remind why (IMHO) we are looking into optimising ATLASDATADISK storage.

From a small site perspective, storage requires a substantial amount of effort to maintain. This effort compared to the volume of storage provided could be efficiently used in other activities. Below is a plot of the percentage of current ATLASDATADISK provided by each site.  The VO also benefits with not using smaller sites as it has fewer logical endpoints to track.
 

This plot shows that if the 10 smaller sites (of which 5 are in the UK) allows for ATLAS to use 99% of space form only 88% of sites. ATLASSCRATCHDISK and ATLASLOCALGROUPDISK usage/requirement also needs to be taken into consideration when deciding if a site should become a fully diskless or caching/buffering site.