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How to access resources? Check out the Xsede user guides for each system in question https://www.xsede.org/web/guest/user-guides Generally speaking most XSEDE systems are configured in a similar fashion and are accessed via ssh like other HPC systems around the world.
Can I login with Tufts credentials ie UTLN? NO. XSEDE does not allow logins using authentication credentials from other institutions though the issue is being examined and may be available in the future.
How to copy files? Similar to using ssh to login, you can use sftp, scp, and rsync over ssh to copy files to a system on XSEDE. If your files are on the Tufts compute cluster they will copy over to XSEDE much faster than from a typical desktop, workstation or laptop on wifi or a wired 1GB network drop. Please feel free to contact Tufts staff to make sure your file copy procedures are as efficient as possible.
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How do I chose between the Tufts HPC (High Performance Compute) cluster and XSEDE? The decision is not mutually exclusive as both clusters are general purpose clusters running a wide variety of software for a an equally broad user base. Typically if you are running custom code it should be compiled individually for each system. Can I login with Tufts credentials ie UTLN? NO. XSEDE does not allow logins using authentication credentials from other institutions though the issue is being examined and may be available in the future.