ATLAS Data Carousel

Abstract

The ATLAS experiment at CERN’s LHC stores detector and simulation data in raw and derived data formats across more than 150 Grid sites world-wide, currently in total about 200PB on disk and 250PB on tape. Data have different access characteristics due to various computational workflows, and can be accessed from different media, such as remote I/O, disk cache on hard disk drives or SSDs. Also, larger data centers provide the majority of offline storage capability via tape systems. For the HighLuminosity LHC (HL-LHC), the estimated data storage requirements are several factors bigger than the present forecast of available resources, based on a flat budget assumption. On the computing side, ATLAS Distributed Computing was very successful in the last years with high performance and high throughput computing integration and in using opportunistic computing resources for the Monte Carlo simulation. On the other hand, equivalent opportunistic storage does not exist. ATLAS started the Data Carousel project to increase the usage of less expensive storage, i.e. tapes or even commercial storage, so it is not limited to tape technologies exclusively. Data Carousel orchestrates data processing between workload management, data management, and storage services with the bulk data resident on offline storage. The processing is executed by staging and promptly processing a sliding window of inputs onto faster buffer storage, such that only a small percentage of input data are available at any one time. With this project, we aim to demonstrate that this is the natural way to dramatically reduce our storage cost. The first phase of the project was started in the fall of 2018 and was related to I/O tests of the sites archiving systems. Phase II now requires a tight integration of the workload and data management systems. Additionally, the Data Carousel studies the feasibility to run multiple computing workflows from tape. The project is progressing very well and the results presented in this document will be used before the LHC Run 3.

Methods of Data Popularity Evaluation in the ATLAS Experiment at the LHC

“…Moreover, a big fraction of files stayed on disk without being accessed for a long time (see Figure 2). The possibility of keeping these files on a less expensive storage media should be investigated further [8].…”

The ATLAS Data Carousel Project Status

“…As explained in our previous paper [8], one important metric in this R&D project is tape efficiency, which is defined by the ratio of the throughput delivered to end users over the vendor-specified nominal throughput of the tape system. It is essential that ATLAS Distributed Computing teams work together with our facilities' tape experts, in order to achieve the optimal usage of our tape resources.…”

“…Phase I and Phase II results have been presented at the CHEP2019 conference [8]. In this manuscript we describe the recent software developments, Phase III accomplishments, and our future plans.…”

dCache: Inter-disciplinary storage system

“…Originally dCache's tertiary connectivity interface was optimized for efficient writing to tapes and file recall was viewed to be a rare event. The recent ATLAS data carousel [19] activities with large-scale recalls have disproved this assumption. Unordered requests to tape libraries resulted in a high number of tape mounts and reposition requests, thus overall low tape access efficiency.…”