CERN Prototype

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protoDUNE

Overview

The protoDUNE experimental program is designed to test and validate the technologies and design that will be applied to the construction of the DUNE Far Detector at the Sanford Underground Research Facility (SURF). The protoDUNE detectors will be run in a dedicated beam line at the CERN SPS accelerator complex. The rate and volume of data produced by these detectors will be substantial and will require extensive system design and integration effort.

As of Fall 2015, "protoDUNE" is the official name for the two apparatuses to be used in CERN beam test: single-phase and dual-phase LArTPC detectors. Each received a formal CERN experiment designation:

The CERN Proposal and the TDR

  • protoDUNE CERN proposal: DUNE DocDB 186
  • as of summer 2016, the protoDUNE TDR is work in progress and is maintained on github. Contact Tom Junk, Brett Viren or Maxim Potekhin for more detail.

Beam, Experimental Hall and other Infrastructure

Materials and Meetings


Expected Data Volume and Rates

Important Note

At the time of writing (Summer 2016) this information is still under development and this page is yet to be edited to reflect the most recent methodology and estimates.

Overview

In order to provide the necessary precision for reconstruction of the ionization patterns in the LArTPC, both single-phase and dual-phase designs share the same fundamental characteristics:

  • High spatial granularity of readout (e.g. the electrode pattern), and the resulting high channel count
  • High digitization frequency (which is essential to ensure a precise position measurement along the drift direction)

Another common factor in both designs is the relatively slow drift velocity of electrons in Liquid Argon, which is of the order of millimeters per microsecond, depending on the drift volume voltage and other parameters. This leads to a substantial readout window (of the order of milliseconds) required to collect all of the ionization in the Liquid Argon volume due the event of interest. Even though the readout times are substantially different in the two designs, the net effect is similar. The high digitization frequency in every channel (as explained above) leads to a considerable amount of data per event. Each event is comparable in size to a high-resolution digital photograph.

There are a few data reduction (compression) techniques that may be applicable to protoDUNE raw data in order to reduce its size. Some of the algorithms are inherently lossy, such as the Zero Suppression algorithm which rejects parts of the digitized waveforms in LArTPC channels according to certain logic (e.g. when the signal is consistently below a predefined threshold for a period of time). There are also lossless compression techniques such as Huffman algorithm and others. At the time of writing it is assumed that only lossless algorithms will be applied to the compression of the protoDUNE raw data.

It is foreseen that the total amount of data to be produced by the protoDUNE detectors will be of the order of a few petabytes (including commissioning runs with cosmic rays). Both instantaneous and average data rates in the data transmission chain are expected to be substantial, e.g. in GB/s for the former and hundreds of MB/s fo the latter. For these reasons, capturing data streams generated by the protoDUNE DAQ systems, buffering of the data, performing fast QA analysis, and transporting the data to sites external to CERN for processing (e.g. FNAL, BNL, etc.) requires significant resources and adequate planning.

Taking data in non-ZS mode

As of Q2 of 2016, non-ZS mode is considered for implementation in protoDUNE (single phase)

Calibration

Software and Computing

Overview

Due to the short time available for data taking, the data to be collected during the experiment is considered "precious" (impossible or hard to reproduce) and redundant storage must be provided for such data. One primary copy would be stored on tape at CERN, another at FNAL and auxiliary copies will be shared between US sites e.g. BNL and NERSC. The aim is to reuse existing software from other experiments to move data between CERN and the US with appropriate degree of automation, error checking and correction, and monitoring.

An effort will be made to implement prompt (near-time) processing and monitoring of data quality, including full tracking in limited express production streams, i.e. on a subset of the data.


DAQ

Raw Data Handling

Conceptual diagram of the data flow is presented below (primarily for raw data). It largely follows the pattern found in the LHC experiments, i.e. contains an online buffer from which the data is transmitted to CERN EOS for further distribution and as an intermediate staging area for tape storage.


ProtoDUNE raw data concept.png

Initial design ideas (as of early 2016) are documented in the paper created during a meeting with FNAL data experts in Mid-March 2016. The main idea is to leverage a few existing systems (F-FTS, SAM etc) in order to satisfy a number of requirements. A few of the initial requirements are documented below:

Online Buffer

Technology Choices

The online buffer is one of the central elements for the raw data management system in protoDUNE. The requirements and initial design ideas are presented in the links above (cf DocDB 1209 and 1212). In terms of implementation, a few options have been considered, e.g. a XROOTD cluster vs a SAN (such as iSCSI or FC).

  • The SAN Technology has the advantage of being available as an appliance, however having looked at the available info it presents its own set of challenges and may not be cost effective after all (see the SAN page for more info).
  • XRootD is a storage clustering technology which does provide scalability in both CPU and storage capacity and throughput which is important for the online buffer proper functioning. There is a "XRootD at BNL" page containing miscellaneous bits of info on how to install and start using XRootD and relevant systems at BNL. The Basic XRootD page is an intro for beginners, which should be helpful to start experimenting with XRootD on your own machine(s).
  • XRootD Buffer is a page detailing a few approaches to the design of the buffer based on XRootD and its interaction with F-FTS.

Online/Offline Interface

The online/offline interface is loosely defined as the middleware located between the Event Builders in the DAQ and the mass storage systems at CERN (such as EOS) and elsewhere (such as dCache at FNAL). As such, the interface has the following principal components:

  • the online buffer
  • F-FTS
  • a notification mechanism which can be as simple as an atomic event leading to creation of a file which is to be "picked up" by the FTS; more elaborate ways exist to notify the FTS.

The leading design version of the buffer is that based on XRootD (see the XRootD Buffer page).

Storage at CERN

More information (including fairly technical bits) can be found on the CERN Data Handling page.

  • EOS is a high-performance distributed disk storage system based on XRootD. It is used by major LHC experiments as the main destination for writing raw data.
  • CASTOR is the principal tape storage system at CERN. It does have a built-in disk layer, which was earlier utilized in production and other activities but this is no longer the case since this functionality is handled more efficiently by EOS. For that reason, the disk storage that exists in CASTOR serves as a buffer for I/O and system functions.


Appendix

Data Characteristics as per the 2015 proposal

This information is presented here mainly for historical reference and to reflect the content of the CERN Proposal.

Estimates were developed under the assumption of Zero Suppression for the single-phase detector. In this case, both data rate and volume are determined primarily by the number of tracks due to cosmic ray muons, recorded within the readout window, which is commensurate with the electron collection time in the TPC (~2ms).

For a quick summary of the data rates, data volume and related requirements see:

A few numbers for the single-phase detector which were used in the proposal:

  • Planned trigger rate: 200Hz
  • Instantaneous data rate in DAQ: 1GB/s
  • Sustained average: 200MB/s

Based on this, the nominal network bandwidth required to link the DAQ to CERN storage elements is ~2GB/s. This is based on the essential assumption that zero suppression will be used in all measurements. There are considerations for taking some portion of the data in non-zs mode, which would require approximately 20GB/s connectivity. Since WA105 specified this as their requirement, DUNE-PT may be able to obtain a link in this range.

The measurement program is still being updated, the total volume of data to be taken will be ~O(1PB). Brief notes on the statistics can be found in Appendix II of the "Materials" page.

Data Transfer: Examples and Reference Materials

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