PC hardware evolves. Applications evolve. Linux evolves. Linux distributions evolve. CERN Linux moves along with the rest of the crowd, trying to match the release schedules of the operating system (OS) and system software (AFS, KDE/GNOME, etc) to our experiments and user-community timelines, and so every 12-24 months a new release needs to be brought forward at CERN. This article looks at the planning for the Linux release that will be used at the LHC start-up.
The CERN Linux currently in distribution is Scientific Linux CERN 3 (SLC3). It is based on Scientific Linux 3 (SL3), an HEP-wide collaboration with FermiLab and CERN as major contributors. SL3 is itself based on the freely available Red Hat Enterprise 3 sources.
SLC3 was certified in November 2004, after six months of validation by the various CERN Linux user communities. SLC3 still supports most of the hardware arriving at CERN, and currently has very few functional deficiencies. But over time, newer hardware will require updates, and newer software versions will offer shiny new functionalities that CERN users may crave. So eventually, SLC3 will need replacing - but when and with what?
In 2007, LHC will start up. Given the complexity of all its computer-driven components, introducing a new Linux release in the middle of start-up is considered unwise. Working backwards from the various deadlines, the last opportunity for changing Linux versions will be around October 2006. Whatever Linux version is certified at that time will stay in place for several years.
Possible candidates are SLC4, derived from Red Hat Enterprise 4 (released in February 2005) via SL4 (released April 2005), and a future SL(C)5 based on the yet-to-come Red Hat Enterprise 5. In the third quarter of 2006, the SLC4 codebase will be about 24 months old (quite old by Linux standards). SLC5 will be fresher (e.g. it will support more recent hardware and have nicer/newer versions of application software), but no firm release date is known. So there is a risk that the formal certification of SLC5 could be delayed beyond the cut-off date in October 2006.
In order to keep SLC5 as an option, but still have a reliable fallback solution in case of delays with it, the LXCERT coordination group decided in May 2005 to fully certify SLC4 until the end of 2005, but not to deploy it widely on CERN machines except for hardware that was too new to be supported by SLC3 (e.g. certain desktop and notebook models).
Assuming its successful certification, SLC4 will be available and supported at CERN during 2006. It will support newer hardware, and may perform better than SLC3 for specific applications. But for the moment it does not have a longer support horizon than SLC3, so should only be deployed in areas where another re-installation a few months later is acceptable.
Instead, an attempt will be made to certify SLC5 in 2006. To speed up the certification process, it has been decided to split the basic OS validation (hardware and typical user applications) from the compiler certification (which concerns mostly the physics software), and merge the OS and compiler late in the process.
Only if SLC5 is delayed will SLC4 become the new production version, and SLC3 will be made obsolete shortly afterwards. If SLC5 certification happens on time, SLC4 will be made obsolete together with SLC3.