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Besides this, the LCG-2 API libraries are also available in the UI for users to link Grid applicacions.
One or more UIs are available at each site part of the LCG-2 Grid.
3.2.3. Computing Element and Storage Element
A Computing Element (CE) is defined as a Grid batch queue and it is identified by a pair
<hostname>/<batch queue name>. A Computing Element is built on a homogeneous farm of computing nodes called Worker Nodes (WN) and a node acting as a Grid Gate (GG) or front-end to the rest
of the Grid. The GG runs a Globus gatekeeper, the Globus GRAM (Globus Resource Allocation Manager) [R9], the master server of a Local Resource Managemente System (LRMS), together with the a
local Logging and Bookkeeping server (see later). In LCG-2 the types of LRMS supported are PBS,
LSF and Condor. While all WNs need only outbound connectivity, the Gate node must be accessible
from outside the site. The GG is responsible for accepting jobs and dispatching them for execution to the
WNs. The GG provides a uniform interface to the computational resources it manages. On the WNs, all
commands and Application Programming Interface (API) for performing actions on Grid resources and
Grid data are available.
Each LCG-2 site runs at least one CE and a farm of WNs behind it.
A Storage Element (SE) provides uniform access and services to large storage spaces. The Storage
Element may control large disk arrays, mass storage systems (MSS) and the like. The current LCG-2
release includes a classic SE, which has a GridFTP server [R11]1 as the data transfer protocol to the
storage resource. It is responsible for secure, fast and efficient file transfer to/from the Storage Element.
In the final LCG-2 release, though, this storage resource will be managed by a Storage Resource
Manager (SRM). This middleware module, will make it possible to dinamically manage the contents of
the storage resource at any time. The SRM will interact with the operating system, with the mass storage
system (to perform file archiving), and with the protocols (to perform file transfer operations).
As MSS, LCG-2 will support d-Cache disk pool (with GridFTP and rfio as transfer protocols), tape
archiving systems, such as Castor (with GridFTP and rfio) and Enstore (with GridFTP). The file protocol
is no longer supported in LCG-2.
Each LCG-2 site provides one or more SEs.
The resources described up to now constitute the compute and storage power of the LCG-2 Grid.
Together with that infrastructure, additional services are provided to locate and report on the status of
Grid resources, to find the most appropriate resources to run a job requiring certain data access and to
automatically perform data operations necessary before and after a job is run. These are the Information
System and the Data Management services.
1 In the literature and throughout this guide the terms GridFTP and gsiftp are used interchangeably to refer to the same secure
grid-enabled ftp protocol.
CERN-LCG-GDEIS-454439
LCG-2 User Guide
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