.. _systems:

Overview of the Systems
==============================

NERSC consists of multiple machines and services, each of which are useful for different tasks.


Connecting
-----------------

Although you can ssh directly to NERSC machines, you will likely have an unpleasant typing experience and may be interrupted by dropped connections, etc.  A much better technique is to download the "NX client" software.  This provides a remote desktop (KDE, just like the late 1990's!) and allows you to disconnect / reconnect without losing work and time.  `Follow instructions here <https://www.nersc.gov/users/connecting-to-nersc/using-nx/>`_.

Within the NX client settings, I suggest clicking on the "Display" options and choosing:

* "Fit to Window"
* Click on "Change settings" and select quality at one step below maximum.  Then change resolution to as low as you can stand (e.g. 1024x600).

These will ensure that you have a small amount of data being sent back and forth over the network, but still have minimal pixelization artifacts.  Such settings usually work well even when tethering to a mobile phone for example.


Cori
-----------------

This sytem has 2 types of nodes:

* `9000 nodes of Intel KNL CPUs <https://www.nersc.gov/users/computational-systems/cori/configuration/cori-intel-xeon-phi-nodes/>`_ with 68 cores per node.  Useful for highly threaded workflows.  Serial code (pure python, outside of numpy/scipy) runs slow.

* `1600 nodes of newer Intel Haswell CPUs <https://www.nersc.gov/users/computational-systems/cori/configuration/cori-phase-i/>`_ with 32 cores per node.  Great for interactive python use.  Good for small jobs.

Edison
----------------

This system has `5500 nodes of older, traditional CPUs <https://www.nersc.gov/users/computational-systems/edison/configuration/>`_, with 2x12 core chips per node.  Good for large jobs that use code which does not run well on the cori KNL nodes.


Disk Space
--------------------

All users have a small home directory for storing things like scripts, git checkouts, etc.  This space is not purged, but is fairly slow.  **Do not read or write large data files here**.

Each individual user has 20TB of personal "scratch" storage.  This location is different on each machine but can always be found in the "$SCRATCH" environment variable.  This disk is very fast, but is purged after some weeks.  Copy any data you want to save to a project directory or your home directory.  You can see your current disk usage with::

    %>  myquota

Each project has shared storage in /project/projectdirs/(project name), which is typically 20TB, but can be increased if we fill that up.  This filesystem is not purged.  You can see current project disk usage with::

    %>  prjquota <project name>

More details on filesystems are `here <https://www.nersc.gov/users/storage-and-file-systems/>`_.


Data Transfer and Backup
--------------------------------

If you are transferring large amounts of data to or from NERSC, and / or backing that data up to HPSS (multi-petabyte tape system), then you should carefully read more about these services:

* `Data transfer nodes <https://www.nersc.gov/users/storage-and-file-systems/data-transfer-nodes/>`_

* `HPSS use <https://www.nersc.gov/users/storage-and-file-systems/hpss/>`_