Apr 1, 2000
The NA48 ´online´ and ´offline´ PC farms
for the NA48 Collaboration
The NA48 experiment at CERN SPS aims to measure the parameter
Re($\epsilon$'/$\epsilon$) of direct CP violation in the neutral kaon system with an accuracy of
0.2 permille. The requirements for the trigger and DAQ system are event rates up to 10 kHz and data
rates up to 150 MByte/s during the 2.5s beam burst.
In 1998 the ´online´ farm with 24 Intel PentiumII based PCs running the LINUX operating system was
designed and built as a memory buffered realtime data acquisition system using Fast Ethernet
technology to cover with an average rate of 32 Mbyte/s. Incoming data is eventwise serialized and
temporarily stored on hard disks. The Online PC farm software guarantees data format integrity and
offers since 1999 a ´self repairing´ and ´hot spare´ mechanism for an almost maintenance free
operation as well as an expert system for data format supervision. The system efficiency in 1999
From the ´online´ PCs raw data is transferred via an optical gigabit link to four SUN disk servers
in the CERN computer center, situated 7km from the NA48 experiment. Arriving data is processed in
realtime with the L3 software trigger/filter running at the ´offline´ PC farm.
The Level 3 software is used as a "step one" online filter performing full reconstruction
and physic analysis for all events with separation into several signal, background and rare decay
streams in a raw and a physics data format for online/offline analysis. In addition the L3 raw data
input stream can be stored.
The infrastructure for a realtime performance is given by a LINUX PC farm with 42 (dual) PentiumII
PCs and the Central Data Recording service of the CERN computer center with four SUN disk servers,
2.5 TB disk space and StorageTek taperobots (50 GB tapes) in a high speed networking environment
using Fast Ethernet and Gigabit Ethernet technology. The I/O peak rate of the disk servers reached
120 MB/s in 1999.
In a second step the prefiltered raw data is reprocessed afterwards with the L3 software to produce
streams in a physics data format for final analysis with refined calibrations.
The raw data volume processed with the ´online´ and ´offline´ PC farm in 1999 was about 100 TByte
collected in 125 days, the physics data output volume in 1999 was on the scale of 3 TByte.