Database Reference
In-Depth Information
not simple and evolved over a period of seven years based on practical expe-
rience and requirements of multiple projects. Yet, it demonstrates that such
a standardization effort to abstract the semantics of storage systems, to pro-
vide a uniform interface to heterogeneous storage systems, and to dynamically
reserve and manage space is possible and valuable, especially as the storage
hardware is changing over time. This effort makes it possible for large projects
to select the functionality they wish to support, regardless of the underlying
storage system. An example of such a large project and its use of SRMs
is presented in the next section. Much of the discussion involves decisions
that had to be made regarding the SRM functionality that the project as a
whole will commit to support, given practical human resource limitations and
deadlines.
3.4 Data Management in WLCG and EGEE
3.4.1 The WLCG Infrastructure
The Worldwide LHC Computing Grid (WLCG) is the largest grid infras-
tructure in operation today, comprising more than 250 sites spread over 45
countries on 5 continents. Its main mission is to provide resources for the
storage and analysis of the data generated by the experiments at the large
hadron collider (LHC) facility currently being commissioned at CERN, the
European Laboratory for Particle Physics. There are four major experiment
at the LHC: a large ion collector experiment (ALICE), a toroidal LHC appa-
ratus (ATLAS), compact muon solenoid (CMS), and LHC beauty experiment
(LHCb). Each experiment has its own computing model, but all rely on the
WLCG for the necessary storage and computing resources. The WLCG itself
comprises a federation of suciently compatible grids. The main contributors
currently are the Enabling Grids for E-sciencE (EGEE)
11
project, the Open
Science Grid (OSG),
12
and the Nordic Data Grid Facility (NDGF).
13
It is
important to note, however, that each of the grids contributing to WLCG
has been explicitly funded to provide infrastructures for e-science in gen-
eral, and in particular for sciences other than particle physics, in contrast
with the main mission of the WLCG. Other disciplines include biomedical re-
search, computational chemistry, nuclear fusion, astronomy, geophysics, me-
teorology, and digital libraries. They usually are new to the ways of work-
ing in large international collaborations that have been normal in particle
physics for decades. While many of the practices used in particle physics
analysis may simply be copied, other disciplines also bring additional require-
ments, in particular with respect to security and privacy. Furthermore, most
WLCG sites also participate in national or other international grids, which
may yet pose other requirements on the services that some of the sites need
to provide. Finally, the large WLCG sites have a history of providing com-
