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Meeting the ALICE data challenge

27 June 2000

Imagine trying to record a symphony in a second. That is
effectively what CERN’s ALICE collaboration will have to do when
the laboratory’s forthcoming Large Hadron Collider (LHC) starts
up in 2005. Furthermore, that rate will have to be sustained for a
full month each year.

ALICE is the LHC’s dedicated
heavy-ion experiment. Although heavy-ion running will occupy just
one month per year, the huge number of particles produced in ion
collisions means that ALICE will record as much data in that month
as the ATLAS and CMS experiments plan to do during the whole
of the LHC annual run. The target is to store one petabyte
(1015 bytes) per year, recorded at the rate of more
than 1 Gbyte/s. This is the ALICE data challenge, and it dwarfs
existing data acquisition (DAQ) applications. At CERN’s current
flagship accelerator LEP, for example, data rates are counted in
fractions of 1 Mbyte/s. Even NASA’s Earth Observing System,
which will monitor the Earth day and night, will take years to
produce a petabyte of data.

Meeting the challenge is a
long-term project, and work has already begun. People from the
ALICE collaboration have been working with members of CERN’s
Information Technology Division to develop the experiment’s data
acquisition and recording systems. Matters are further complicated
by the fact that the ALICE experiment will be situated several
kilometres away from CERN’s computer centre, where the data will
be recorded. This adds complexity and makes it even more
important to start work now.

Standard components – such as
CERN’s network backbone and farms of PCs running the Linux
operating system – will be used to minimize capital outlay. They
will, however, be reconfigured for the task in order to extract the
maximum performance from the system. Data will be recorded by
StorageTek tape robots installed as part of the laboratory’s
tape-automation project to pave the way for handling the large
number of tapes that will be required by LHC
experiments.

The first goal for the ALICE data challenge was
to run the full system at a data transfer rate of 100 Mbyte/s – 10%
of the final number. This was scheduled for March and April 2000
so as not to interfere with CERN’s experimental programme, which
will get up to speed in the summer.

Data sources for the test
were simulated ALICE events from a variety of locations at CERN.
After being handled by the ALICE DAQ system (DATE) they
were formatted by the ROOT software, developed by the global
high-energy physics community. The data were then sent through
the CERN network to the computer centre, where two mass
storage systems were put through their paces for two weeks each.
The first, HPSS, is the fruit of a collaboration between industry and
several US laboratories. The second, CASTOR, has been developed
at CERN.

Although each component of the system had been
tested individually and shown to work with high data rates, this
year’s tests have demonstrated the old adage that the whole is
frequently greater than the sum of its parts: problems only arose
when all of the component systems were integrated.

The tests
initially achieved a data rate of 60 Mbyte/s with the whole chain
running smoothly. However, then problems started to appear in the
Linux operating system used in the DAQ system’s PC farms.
Because Linux is not a commercial product, the standard way of
getting bugs fixed is to post a message on the Linux newsgroups.
However, no-one has previously pushed Linux so hard, so solutions
were not readily forthcoming and the team had to work with the
Linux community to find their own.

That done, the rate was
cranked up and failures started to occur in one of the CERN
network’s many data switches. These were soon overcome – thanks
this time to an upgrade provided by the company that built the
switches – and the rate was taken up again. Finally the storage
systems had trouble absorbing all of the data. When these problems
were ironed out, the target peak rate of 100 Mbyte/s was achieved
for short periods.

At the end of April the ALICE data
challenge team had to put their tests on hold, leaving the CERN
network and StorageTek robots at the disposal of ongoing
experiments and test beams. During the tests, more than 20 Tbyte
of data – equivalent to some 2000 standard PC hard disks – had
been stored. The next milestone, scheduled for 2001, is to run the
system at 100 Mbyte/s in a sustained way before increasing the rate,
step by step, towards the final goal of 1 Gbyte/s by 2005. The
ALICE data challenge team may not yet have made a symphony,
but the overture is already complete.

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