Database Reference
In-Depth Information
at FNAL, the Tier-1 center where the dCache SRM is developed. The major-
ity of the Tier-1 centers have since adopted dCache as MSS front-end. In the
autumn of 2004 the CASTOR services at CERN and three Tier-1 centers also
became accessible through SRM v1.1, while retaining a classic SE appearance
in parallel for backward compatibility. In the spring of 2005, to assist in the
configuration and operation of Tier-2 sites, the WLCG/EGEE middleware
releases started including support for both dCache and Disk Pool Manager
(DPM) installations. dCache has more options for advanced configurations
(e.g., separation of read and write pools), but the DPM has been simpler to
operate. In early 2008 CASTOR is in use at 7 WLCG sites, dCache at about
60, and the DPM at about 130.
Though the transition to SRM v1.1 has brought significant improvements to
WLCG data management, it became clear that it still had important defects:
SRM v1.1 still lacks advance space reservation. It only allows for an implicit
space reservation as the first part of a short-lived store operation. This
does allow for the operation to be canceled cleanly when insucient
space happens to be available, though.
SRM v1.1 lacks an elegant prestaging functionality. When a file has to be
recalled from tape, the client will either have to remain connected and
wait, or it would have to resort to a server-specific protocol for having
the file staged in advance.
There is no portable way to guarantee the removal of files that are no longer
wanted. SRM v1.1 only has an advisory delete function, whose effects
differ in different implementations. Client tools typically have to recog-
nize the various implementations and invoke server-specific algorithms,
contrary to the idea of a protocol standard.
SRM v1.1 lacks equivalents to basic file system operations for, for example,
renaming files, removing directories, or changing permissions. Directo-
ries are created implicitly.
As for the Classic SE, files and directories owned by a VO have to be
made writable for their whole VO by default, so that any member of the VO
can write to the SE without further administrative operations or requiring a
strict organization of the VO name space. Exceptions are made for the VO
production managers, who are responsible for the vast majority of the data
produced by a VO. Usually they ask for dedicated subsets of the VO name
space, where only they can write. At the same time they can negotiate the
desired quality of service, for example, dedicated disk pools.
3.4.4.2
The Storage Element Service
In the first quarter of 2005 the WLCG Baseline Services working group
(BSWG) 25 was established in order to understand the experiment require-
ments for their data challenges. For each of the experiments a data challenge
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